Object table comprising an electrostatic clamp

ABSTRACT

Disclosed is an object table for holding an object, comprising: an electrostatic clamp arranged to clamp the object on the object table; a neutralizer arranged to neutralize a residual charge of the electrostatic clamp; a control unit arranged to control the neutralizer, wherein the residual charge is an electrostatic charge present on the electrostatic clamp when no voltage is applied to the electrostatic clamp.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of International applicationPCT/EP2019/085233, which claims priority to EP application 18214362.8,which was filed on Dec. 20, 2018, and to EP application 19171929.3,which was filed on Apr. 30, 2019, and to U.S. application 62/946,340,which was filed on Dec. 10, 2019. Each of these aforementionedapplications are incorporated herein by reference in their entireties.

FIELD

The embodiments of the present disclosure relate to an object table, inparticular an object table as can be applied in an inspection apparatussuch as a particle beam inspection apparatus.

BACKGROUND ART

The embodiments of the present disclosure relate to an object table, inparticular an object tables as can be applied in an inspection apparatussuch as a particle beam inspection apparatus. Such inspectionapparatuses can e.g., be applied for the inspection of objects such assemiconductor substrates, also referred to as wafers, that are appliedin lithographic processes. Such inspection apparatuses may also beapplied for the inspection of patterning devices, also referred to asreticles.

In semiconductor processes, defects may be generated that may impactdevice performance and even result in device failure. Device yield maythus be impacted, resulting in cost raise. In order to controlsemiconductor process yield, defect monitoring is important. One tooluseful in defect monitoring is an electron beam inspection system, suchas a SEM (Scanning Electron Microscope), which scans a target portion ofa specimen using one or more beams of electrons.

During operation of an inspection tool, the substrate is typically heldby an object table. The inspection tool will typically comprise asubstrate positioning device for positioning the object table, while thesubstrate is held by the object table, relative to the particle beamsuch as an e-beam, in order to position a target area on the substrate,i.e., an area that needs to be inspected, in an operating range of thee-beam. Such a substrate positioning device may e.g., comprise aplurality of actuators and motors for realizing the requiredpositioning.

The substrate positioning device e.g., comprises a first part supportingthe substrate, for example with an object table of the first part, and asecond part that movably supports the first. In these embodiments,movement of the first part with respect to the second part is realizedby placing two linear actuator systems on top of each other. The firstactuator system is arranged to provide a movement in a first horizontaldirection and a second actuation system, supported on the firstactuation system is arranged to provide a movement in a secondhorizontal direction, perpendicular to the first horizontal direction.

The second part supports a short-stroke actuator system that allows toposition the object table supporting the substrate in three degrees offreedom, i.e., the vertical direction and rotations about the first andsecond horizontal directions. This short-stroke position system enableslevelling of the substrate in the focal point of the inspection beam.

The inspection E-beam can be manipulated in the first and secondhorizontal direction by means of a deflection unit in the inspectiontool. This functionality may be used for fine positioning of theinspection beam relative to the substrate.

In order to ensure that the object, e.g., the substrate, is maintainedat a desired position during the inspection process, the object table istypically configured to apply a clamping force on the object. In orderto achieve this, the object table as applied in the inspection apparatusmay e.g., comprise an electrostatic clamp that is configured to apply aholding or clamping force onto the object. Such an electrostatic clampmay typically have one or more electrodes, e.g., embedded in adielectric material. Further, object tables that are used in inspectionapparatuses such as particle beam apparatuses may be equipped with anelectrode, also referred to as a high-voltage electrode, that isconfigured to generate an appropriate electric field for the particlebeam during the inspection of the object. The application of anelectrostatic clamp for holding the object that is to be inspected maypose some problems. In particular, when an electrostatic clamp is usedto clamp objects, a charge may build up on a surface of the clamp, whichrenders the unloading of clamped objects more difficult, i.e., theobjects tend to stick to the clamp, even when no voltage is applied tothe clamp.

In addition, during the unloading process of an object, there is a riskof sparking or discharging towards the high-voltage electrode or otherparts of the object table.

SUMMARY

One of the objects of the present disclosure is to provide an objecttable for use in an inspection apparatus in which the aforementionedproblems are at least mitigated. Such inspection apparatuses can e.g.,be applied for the inspection of objects such as semiconductorsubstrates, also referred to as wafers, that are applied in lithographicprocesses. Such inspection apparatuses may also be applied for theinspection of patterning devices, also referred to as reticles. Theinspection apparatuses in which the object table according to thepresent disclosure is applied may advantageously also be applied forprocess control of processes such as lithographic processes. In sucharrangements, the inspection apparatuses may e.g., be applied to detect,by inspecting the object, defects on the object, e.g., a substrate, orto assess process parameters such as illumination settings, appliedillumination dose, etc. as applied in a lithographic processing of theobject. The parameters as determined may then be applied as feedback toadjust the lithographic process.

According to some embodiments of the present disclosure, there isprovided an object table comprising:

-   -   a clamping mechanism for holding an object;    -   a loading/unloading mechanism configured to contact the object        to load or unload the object;    -   an electrical conductor configured to electrically connect the        object to a voltage source or an electrical ground to apply a        predetermined voltage to the object during at least part of an        unloading sequence of the object,        wherein the electrical conductor is configured to form a low        mechanical stiffness connection when the object is held on the        object table.

According to some embodiments of the present disclosure, there isprovided an object table for use in an inspection apparatus, the objecttable being configured to hold an object such as a substrate andcomprising:

-   -   an electrostatic clamp configured to hold the object;    -   a measurement unit configured to determine an electric        characteristic of the electrostatic clamp, the electric        characteristic being representative of a charge state of the        electrostatic clamp;    -   a control unit configured to control, during an unloading of the        object, a power supply of the electrostatic clamp, based on the        determined electric characteristic.

According to some embodiments of the present disclosure, there isprovided an object table for holding an object, comprising:

-   -   an electrostatic clamp arranged to clamp the object on the        object table; a neutralizer arranged to neutralize a residual        charge of the electrostatic clamp;    -   a control unit arranged to control the neutralizer,        wherein the residual charge is an electrostatic charge present        on the electrostatic clamp when no voltage is applied to the        electrostatic clamp.

According to some embodiments of the present disclosure, there isprovided a method for clamping an object on an electrostatic clamp, themethod comprising:

-   -   i) providing the object on the electrostatic clamp;    -   ii) increasing a clamping voltage until a clamped state is        detected in which the object is clamped on the electrostatic        clamp;    -   iii) determining a first clamping voltage (V_(max)) being the        clamping voltage at the clamped state;    -   iv) providing a second clamping voltage (V_(final)) to the        electrostatic clamp, which is less than the first clamping        voltage (V_(max)).

According to some embodiments of the present disclosure, there isprovided a method of determining a residual charge of a clampingmechanism of an object table, the method comprising:

-   -   impinging a surface of the clamping mechanism using a particle        beam;    -   detecting a response of the clamping mechanism caused by the        impinging of the surface, and    -   determining the residual charge of the clamping mechanism, based        on the response.

According to some embodiments of the present disclosure, there isprovided a particle beam apparatus comprising:

-   -   a particle beam generator;    -   an object table for holding an object, the object table        comprising a clamping mechanism for clamping the object to the        object table;    -   a detector;    -   a control unit, the control unit being configured to:        -   control the particle beam generator to cause a particle beam            to impinge on a surface of the clamping mechanism;    -   the detector being configured to detect a response of the        clamping mechanism, caused by the clamping mechanism being        impinged by the particle beam;    -   the control unit further being configured to:        -   receive a detector signal from the detector, the detector            signal representing the response of the clamping mechanism;        -   determine a residual charge on the clamping mechanism, based            on the detector signal.

According to some embodiments of the present disclosure, there isprovided a method of reducing a surface charge of a clamping mechanism,the method comprising:

-   -   generating a particle beam, the particle beam being configured        to have a secondary emission yield (SEY) substantially equal to        1 in a surface of the clamping mechanism;    -   impinging the surface of the clamping mechanism using the        particle beam.

According to some embodiments of the present disclosure, there isprovided a particle beam apparatus comprising:

-   -   a particle beam generator;    -   an object table for holding an object, the object table        comprising a clamping mechanism for clamping the object to the        object table;    -   a control unit, the control unit being configured to:        -   control the particle beam generator to generate a particle            beam, the particle beam being configured to have a secondary            emission yield (SEY) substantially equal to 1 in a surface            of the clamping mechanism;        -   control the particle beam to impinge the surface of the            clamping mechanism.

According to some embodiments of the present disclosure, there isprovided an object table comprising:

-   -   an electrostatic clamp arranged to clamp the object on the        object table; and    -   a cleaning device;        wherein the cleaning device is arranged to clean the        electrostatic clamp.

According to some embodiments of the present disclosure, there isprovided an object table comprising:

-   -   an electrostatic clamp arranged to clamp the object on the        object table; and    -   one or more electrodes arranged to charge the object;        wherein a first set of the one or more electrodes is arranged to        apply an electric charge to the object; and a second set of the        one or more electrodes is arranged to electrically discharge the        object.

According to some embodiments of the present disclosure, there isprovided an object table for holding an object comprising:

-   -   an electrostatic clamp arranged to clamp the object on the        object table;    -   one or more elevation pins arranged to lift the object up from        the object table; and    -   a controller configured to send an actuation signal to one or        more elevation pin positioning device so as to vibrate the one        or more elevation pins and/or at least part of the object table.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be readily understood by the followingdetailed description in conjunction with the accompanying drawings,wherein like reference numerals designate like structural elements, andin which:

FIG. 1a and 1b are schematic illustrations of an e-beam inspection toolaccording to some embodiments of the present disclosure.

FIGS. 2 and 3 are schematic illustrations an electron optical system ascan be applied in some embodiments of the present disclosure.

FIG. 4 schematically depicts a possible control architecture of an EBIsystem according to some embodiments of the present disclosure.

FIG. 5 schematically shows a cross-sectional view of an object table.

FIG. 6 schematically shows a cross-sectional view of another objecttable.

FIGS. 7a and 7b schematically show a first arrangement of an electricalconductor, according to some embodiments of the present disclosure.

FIG. 7c schematically show another arrangement of an electricalconductor, according to some embodiments of the present disclosure.

FIG. 8 schematically shows a cross-sectional view of a second objecttable, according to some embodiments of the present disclosure.

FIG. 9 schematically shows a second arrangement of an electricalconductor, according to some embodiments of the present disclosure.

FIG. 10 schematically shows a third arrangement of an electricalconductor, according to some embodiments of the present disclosure.

FIGS. 11a and 11b schematically shows a fourth arrangement of anelectrical conductor, according to some embodiments of the presentdisclosure.

FIG. 12 schematically shows a fifth arrangement of an electricalconductor, according to some embodiments of the present disclosure.

FIG. 13 schematically shows a cross-sectional view of a third objecttable, according to some embodiments of the present disclosure.

FIG. 14 schematically shows a cross-sectional view of a fourth objecttable, according to some embodiments of the present disclosure.

FIG. 15 schematically shows a cross-sectional view of a fifth objecttable, according to some embodiments of the present disclosure.

FIG. 16 schematically shows a further example of an object table,according to some embodiments of the present disclosure.

FIG. 17 schematically presents a clamping voltage as provided to theelectrostatic clamp according to some embodiments of the presentdisclosure.

FIG. 18 schematically presents a clamping voltage as provided to theelectrostatic clamp according to some embodiments of the presentdisclosure.

FIG. 19 schematically presents a clamping voltage as provided to theelectrostatic clamp according to some embodiments of the presentdisclosure.

FIG. 20 schematically presents yet another example of an object table,according to some embodiments of the present disclosure.

FIG. 21 schematically shows a flowchart of a method of determining theresidual charge of a clamping mechanism, according to some embodimentsof the present disclosure.

FIG. 22 schematically shows a particle beam apparatus according to someembodiments of the present disclosure.

FIG. 23 schematically shows a flowchart of a method of reducing asurface charge of a clamping mechanism.

FIG. 24 schematically shows an SEY vs. LE graph.

FIG. 25 schematically shows a particle beam apparatus, according to someembodiments of the present disclosure.

FIG. 26 is a conceptual figure that shows an object on an object table,according to some embodiments of the present disclosure.

FIG. 27 is a conceptual figure that shows an object on an object table,a positioning device and an object unloading device.

FIG. 28 is a conceptual figure that shows an object on an object table,according to some embodiments of the present disclosure.

FIG. 29 is a conceptual figure that shows an object on an object table,according to some embodiments of the present disclosure.

While the disclosed embodiments are susceptible to various modificationsand alternative forms, specific embodiments thereof are shown by way ofexample in the drawings and may herein be described in detail. Thedrawings may not be to scale. It should be understood, however, that thedrawings and detailed description thereto are not intended to limit thedisclosure to the particular form disclosed, but on the contrary, theintention is to cover all modifications, equivalents and alternativesfalling within the spirit and scope of the embodiments of the presentdisclosure as defined by the appended claims.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Various example embodiments of the present disclosure will now bedescribed more fully with reference to the accompanying drawings inwhich some example embodiments of the present disclosure are shown. Inthe drawings, the thicknesses of layers and regions may be exaggeratedfor clarity.

Detailed illustrative embodiments of the present disclosure aredisclosed herein. However, specific structural and functional detailsdisclosed herein are merely representative for purposes of describingexample embodiments of the present disclosure. These embodiments may,however, be embodied in many alternate forms and should not be construedas limited to only the embodiments set forth herein.

Accordingly, while example embodiments of the present disclosure arecapable of various modifications and alternative forms, embodimentsthereof are shown by way of example in the drawings and will herein bedescribed in detail. It should be understood, however, that there is nointent to limit example embodiments of the present disclosure to theparticular forms disclosed, but on the contrary, example embodiments ofthe present disclosure are to cover all modifications, equivalents, andalternatives falling within the scope of the present disclosure. Likenumbers refer to like elements throughout the description of thefigures.

As used herein, the term “specimen” generally refers to a wafer or anyother specimen on which defects of interest (DOI) may be located.Although the terms “specimen” and “sample” are used interchangeablyherein, it is to be understood that embodiments described herein withrespect to a wafer may configured and/or used for any other specimen(e.g., a reticle, mask, or photomask).

As used herein, the term “wafer” generally refers to substrates formedof a semiconductor or non-semiconductor material. Examples of such asemiconductor or non-semiconductor material include, but are not limitedto, monocrystalline silicon, gallium arsenide, and indium phosphide.Such substrates may be commonly found and/or processed in semiconductorfabrication facilities.

The term, crossover, refers to a point where the electron beam isfocused.

The term, virtual source, means the electron beam emitted from thecathode can be traced back to a “virtual” source.

The inspection tool according to the present disclosure may relate to acharged particle source, especially to an e-beam source which can beapplied to a SEM, an e-beam inspection tool, or an EBDW. The e-beamsource, in this art, may also be referred to as an e-gun (Electron Gun).

With respect to the drawings, it is noted that the figures are not drawnto scale. In particular, the scale of some of the elements of thefigures may be greatly exaggerated to emphasize characteristics of theelements. It is also noted that the figures are not drawn to the samescale. Elements shown in more than one figure that may be similarlyconfigured have been indicated using the same reference numerals.

In the drawings, relative dimensions of each component and among everycomponent may be exaggerated for clarity. Within the followingdescription of the drawings the same or like reference numbers refer tothe same or like components or entities, and only the differences withrespect to the individual embodiments are described.

Accordingly, while example embodiments of the present disclosure arecapable of various modifications and alternative forms, embodimentsthereof are shown by way of example in the drawings and will herein bedescribed in detail. It should be understood, however, that there is nointent to limit example embodiments of the present disclosure to theparticular forms disclosed, but on the contrary, example embodiments ofthe present disclosure are to cover all modifications, equivalents, andalternatives falling within the scope of the present disclosure.

FIGS. 1a and 1b schematically depict a top view and a cross-sectionalview of an electron beam (e-beam) inspection (EBI) system 100 which maye.g., be according to some embodiments of the present disclosure. Theexample as shown comprises an enclosure 110, a pair of load ports 120serving as an interface to receive objects to be examined and to outputobjects that have been examined. The example as shown further comprisesan object transfer system, referred as an EFEM, equipment front endmodule 130, that is configured to handle and/or transport the objects toand from the load ports. In the example as shown, the EFEM 130 comprisesa handler robot 140 configured to transport objects between the loadports and a load lock 150 of the EBI system 100. The load lock 150 is aninterface between atmospheric conditions occurring outside the enclosure110 and in the EFEM and the vacuum conditions occurring in a vacuumchamber 160 of the EBI system 100. In the example as shown, the vacuumchamber 160 comprises an electron optics system 170 configured toproject an e-beam onto an object to be inspected, e.g., a semiconductorsubstrate or wafer. The EBI system 100 further comprises a positioningdevice 180 that is configured to displace the object 190 relative to thee-beam generated by the electron optics system 170.

In some embodiments, the positioning device may comprise a cascadedarrangement of multiple positioners such an XY-stage for positioning theobject in a substantially horizontal plane, and a Z-stage forpositioning the object in the vertical direction.

In some embodiments, the positioning device may comprise a combinationof a coarse positioner, configured to provide a coarse positioning ofthe object over comparatively large distances and a fine positioner,configured to provide a fine positioning of the object overcomparatively small distances.

In some embodiments, the positioning device 180 further comprises anobject table for holding the object during the inspection processperformed by the EBI system 100. In such an example, the object 190 maybe clamped onto the object table by means of a clamp such as anelectrostatic clamp. Such a clamp may be integrated in the object table.

In some embodiments, the positioning device 180 comprises a firstpositioner for positioning the object table and a second positioner forpositioning the first positioner and the object table. In addition, thepositioning device 180 as applied in the e-beam inspection tool 100 maycomprise a heating device that is configured to generate a heat load inthe object table.

FIG. 2 schematically depict an example of an electron optics system 200as can be applied in e-beam inspection tool or system according to thepresent disclosure. The electron optics system 200 comprises an e-beamsource, referred to as the electron gun 210 and an imaging system 240.

The electron gun 210 comprises an electron source 212, suppressor 214,an anode 216, a set of apertures 218, and a condenser 220. The electronsource 212 can be a Schottky emitter. More specifically, in someembodiments the electron source 212 includes a ceramic substrate, twoelectrodes, a tungsten filament, and a tungsten pin. The two electrodesare fixed in parallel to the ceramic substrate, and the other sides ofthe two electrodes are respectively connected to two ends of thetungsten filament. The tungsten is slightly bended to form a tip forplacing the tungsten pin. Next, a ZrO2 is coated on the surface of thetungsten pin and is heated to 1300° C. so as to be melted and cover thetungsten pin but uncover the pinpoint of the tungsten pin. The meltedZrO2 can make the work function of the tungsten lowered and decrease theenergy barrier of the emitted electron, and thus the electron beam 202is emitted efficiently. Then, by applying negative electricity to thesuppressor 214, the electron beam 202 is suppressed. Accordingly, theelectron beam having the large spread angle is suppressed to the primaryelectron beam 202, and thus the brightness of the electron beam 202 isenhanced. By the positive charge of the anode 216, the electron beam 202can be extracted, and then the Coulomb's compulsive force of theelectron beam 202 may be controlled by using the tunable aperture 218which has different aperture sizes for eliminating the unnecessaryelectron beam outside of the aperture. In order to condense the electronbeam 202, the condenser 220 is applied to the electron beam 202, whichalso provides magnification. The condenser 220 shown in the FIG. 2 maye.g., be an electrostatic lens which can condense the electron beam 202.On the other hand, the condenser 220 can be also a magnetic lens.

The imaging system 240 as shown in FIG. 3 comprises a blanker 248, a setof apertures 242, a detector 244, four sets of deflectors 250, 252, 254,and 256, a pair of coils 262, a yoke 260, a filter 246, and an electrode270. The electrode 270 is used to retard and deflect the electron beam202, and further has electrostatic lens function due to the combinationof upper pole piece and sample 300. Besides, the coil 262 and the yoke260 are configured to the magnetic objective lens.

The electron beam 202, described above, is generated by heating theelectron pin and applying the electric field to anode 216, so that, inorder to stabilize the electron beam 202, there must be a long time forheating the electron pin. For a user end, it is surely time consumingand inconvenient. Hence, the blanker 248 is applied to the condensedelectron beam 202 for temporally deflecting the electron beam 202 awayfrom the sample rather than turning off it.

The deflectors 250 and 256 are applied to scan the electron beam 202 toa large field of view, and the deflectors 252 and 254 are used forscanning the electron beam 202 to a small field of view. All thedeflectors 250, 252, 254, and 256 can control the scanning direction ofthe electron beam 202. The deflectors 250, 252, 254, and 256 can beelectrostatic deflectors or magnetic deflectors. The opening of the yoke260 is faced to the sample 300, which immerses the magnetic field intothe sample 300. On the other hand, the electrode 270 is placed beneaththe opening of the yoke 260, and therefore the sample 300 will not bedamaged. In order to correct the chromatic aberration of the electronbeam 202, the retarder 270, the sample 300, and the upper pole pieceform a lens to eliminate the chromatic aberration of the electron beam202.

Besides, when the electron beam 202 bombards into the sample 300, asecondary electron will be emanated from the surface of the sample 300.Next the secondary electron is directed to the detector 244 by thefilter 246.

FIG. 4 schematically depicts a possible control architecture of an EBIsystem according to the present disclosure. As indicated in FIG. 1, theEBI system comprises a load lock, a wafer transfer system, a load/lock,an electron optics system and a positioning device, e.g., including az-stage and an x-y stage. As illustrated, these various components ofthe EBI system may be equipped with respective controllers, i.e., awafer transporter system controller connected to the wafer transfersystem, a load/lock controller, an electron optics controller, adetector controller, a stage controller. These controllers may e.g., becommunicatively connected to a system controller computer and an imageprocessing computer, e.g., via a communication bus. In the example asshown, the system controller computer and the image processing computermay be connected to a workstation.

The load port loads a wafer to the wafer transfer system, such as EREM130, and the wafer transfer system controller controls the wafertransfer to transfer the wafer to the load/lock, such as load lock 150.The load/lock controller controls the load/lock to the chamber, suchthat an object that is to be examiner, e.g., a wafer can be fixed on aclamp, e.g., an electrostatic clamp, also referred to as an e-chuck. Thepositioning device, e.g., the z-stage and the x-y stage, enable thewafer to move by the stage controller. In some embodiments, a height ofthe z-stage may e.g., be adjusted using a piezo component such as apiezo actuator. The electron optic controller may control all theconditions of the electron optics system, and the detector controllermay receive and convert the electric signals from the electron opticsystem to image signals. The system controller computer is to send thecommands to the corresponding controller. After receiving the imagesignals, the image processing computer may process the image signals toidentify defects.

As mentioned above, during the inspection, the object is held on anobject table by means of a clamp or clamping arrangement. Such a clampor clamping arrangement may e.g., comprise an electrostatic clamp. Suchan electrostatic clamp may e.g., comprise one or more electrodesconfigured to generate an electrostatic field that causes an attractiveforce between the object, e.g., a substrate, and the clamp. As such,during the inspection process, when the object table may be displacedrelative to the inspection beam of radiation, the object can be held ata fixed position on the object table.

In general, the process of inspecting an object such as a substrate mayinvolve the following steps:

In a first step, an object that is to be inspected, is brought in thevicinity of the object table. This can e.g., be done using a robot orhandler. Such a robot or handler may e.g., be configured to position theobject above the object table, in particular above a support/clampingsurface of the object table.

In a second step, the object is mounted to the object table, e.g., ontothe support/clamping surface of the object table. This step can e.g., berealized by means of a loading/unloading mechanism of the object table.In some embodiments, such a loading/unloading mechanism may comprise oneor more pin-shaped members that can protrude through the object table,support the object and lower the object onto the supporting surface ofthe object table.

In a third step, once the object is mounted onto the object table, theclamp, e.g., the electrostatic clamp, may be operated in order to clampthe object to the support surface of the object table.

In a fourth step, the inspection process can be performed, during whichthe object may e.g., be subjected to an inspection beam such as aparticle beam, e.g., an electron beam. During such inspection process,the object table and the object as held, can e.g., be displaced relativeto the inspection beam by means of a positioner as described above.

In a fifth step, the object may be released, e.g., by de-energizing theclamp or clamp arrangement.

In a sixth step, the object may then be moved away from the supportsurface, e.g., lifted by the loading/unloading mechanism, in order to bereceived by a robot or handler, which may then remove the inspectedsubstrate from the inspection apparatus.

It has been observed by the inventors that there may be one or moreissues or problems associated with the aforementioned process.

In particular, the following issue may arise when a conventional objecttable is used in an inspection apparatus during the inspection processas mentioned:

The issue or problem as mentioned relates in particular to the unloadingprocess of the object after the inspection process has been performed.In particular, it has been observed that, when an electrostatic clamp isused to clamp the object, charges may build up on a surface of theelectrostatic clamp. Such a build-up of charges may occur gradually,e.g., during the processing of multiple objects. As a result of thissurface charge, a voltage of the object may increase when the object islifted from the object table. This increase of the voltage of the objectmay cause an electric discharge between the object and the surface ofthe electrostatic clamp or surroundings, damaging the object, theelectrostatic clamp, or the surrounding, or causing contamination of thespace around the object and the electrostatic clamp. This may be anissue, especially for any vacuum apparatus where the object and theelectrostatic clamp are arranged in a vacuum chamber. Prior to theunloading of the object, the object may e.g., be inspected using aparticle beam such as an electron beam. During such inspection, anelectrode of the object table, e.g., a high voltage electrode mounted tothe object table, may be connected to a voltage source, as well as theobject itself. When the object needs to be unloaded, the high voltageelectrode may e.g., be grounded and the object is disconnected from theelectrode and unloaded by means of a loading/unloading mechanism whichcan e.g., lift or lower the object from the object table. The remainingsurface charge on the electrostatic clamp during the unloading processmay also cause an attracting force between the object and theelectrostatic clamp. Required force for unloading the object by anunloading mechanism may increase or even the unloading mechanism may notbe able to lift and unload the object.

In known arrangements, the lifting or lowering mechanism of an objecttable will in general be made from electrically insulating members, inorder to ensure that no sparking or discharging towards the membersoccurs during the inspection process. It has been observed by theinventors that the increase of the voltage of the object occurringduring the unloading of the object may be such that it causes adischarging or sparking, e.g., towards the high-voltage electrode whichis typically grounded during the unloading process.

An object table that may suffer from the above-mentioned issue orproblem is schematically shown in FIG. 5.

FIG. 5 schematically shows a cross-sectional view of an object table asknown in the art. FIG. 5 schematically shows a cross-sectional view ofan object table 500, the object table 500 comprising a support member510. The object table 500 further comprises an electrostatic clamp 530that is arranged in a recess of the support member, the electrostaticclamp 530 having a support surface 510.1 for supporting an object 520,e.g., a semiconductor substrate. Such an electrostatic clamp may beprovided with one or more electrodes which can be connected to a voltagesupply. The object table 500 further comprises an electrode 540, theelectrode 540 surrounding the support surface 510.1 onto which theobject 520 is mounted during the inspection. In a particle beaminspection apparatus, such an electrode 540 may e.g., be applied togenerate a suitable electric field for inspecting the object. In anelectron beam inspection apparatus, the electrode may e.g., be connectedto a negative voltage source during the inspection of the object 520.The object table 500 further comprises a loading/unloading mechanism 550comprising pin-shaped members 550.1. During use, the members 550.1 canbe displaced in vertical direction by means of an actuator 550.2, thusenabling the object 520 to be lifted from the support surface 510.1(i.e., unloaded the object) or enabling the object 520 to be loweredonto the support surface 510.1 (i.e., loading the object). In thearrangement as shown, the actuator 550.2 may e.g., be mounted to apositioning device 560 configured to position the object table 500.Typically, the positioner and the actuator 550.2 would be grounded.Since the electrostatic clamp 530 and the electrode 540 may be atcomparatively high voltages during operation, the pin-shaped members550.1 are typically made from electrically isolated materials, in orderto avoid a discharging or sparking. The known object table 500 asschematically shown in FIG. 5 may suffer from the above-mentioned issue.This can be illustrated as follows:

During the clamping of objects such as semiconductor substrates, agradual build-up or accumulation of charges may occur on the surface ofthe electrostatic clamp 530. In FIG. 5, such an accumulated charge, alsoreferred to as surface charge, is indicated by + and − signs 530.3. Inthe arrangement as shown, the surface charge is generated on a surfaceof the electrostatic clamp 530 that faces a bottom surface of the object520. As a result of this surface charge, a voltage will be induced inthe object 520 when the object 520 is lifted from the support surface510.1, i.e., when the object is unloaded. This voltage is caused by thefact that the object is insulated from the electrostatic clamp, i.e., ata floating potential. When, during the unloading, the object is liftedfrom the support surface by means of the insulated pin-shaped members550.1, a voltage is induced in the object 520 due to the varyingcapacitance formed by the bottom surface 520.1 of the object 520 and thesurface of the electrostatic clamp that is provided with the surfacecharge 530.3. As the distance between both surfaces increases, thecapacitance value decreases, causing an increase of the voltage on theobject 520. This increased voltage on the object may cause a spark 570from the object 520 to a nearby conductive surface, e.g. to theelectrode 540.

In order to mitigate this sparking problem, it has been proposed toprovide an electrical connection between the electrode of the objecttable and a top portion or tip of the loading/unloading mechanism. Sucha known arrangement is schematically shown in FIG. 6.

FIG. 6 schematically shows a cross-sectional view of an object table asknown in the art. FIG. 6 schematically shows a cross-sectional view ofan object table 600, the object table 600 comprising, similar to theobject table 500 of FIG. 5, a support member 510 and an electrostaticclamp 530 that provides a support surface 510.1 for supporting an object520, e.g., a semiconductor substrate. In the arrangement as shown, theelectrostatic clamp 530 is arranged in a recess of the support member510. In the arrangement as shown, the electrostatic clamp is be providedwith one or more clamping electrodes 530.1 which can be connected to avoltage supply (not shown). In the arrangement as shown, the objecttable 600 further comprises an electrode 540 arranged adjacent thesupport surface 510.1. During use, the electrode 540 may e.g., beconnected to a voltage source 542, e.g., a negative voltage source. Inthe arrangement as shown, the voltage source 542 is schematicallyrepresented by an output terminal 542.1 via which a suitable voltage canbe applied to the electrode 540. The electrode 540, which may also bereferred to as the high-voltage electrode 540 may be, during inspection,connected to a suitable voltage such that a substantially uniformelectric potential is produced around the object 520. The object table600 further comprises a loading/unloading mechanism 550 comprisingpin-shaped members 550.1. During use, the members 550.1 can e.g., bedisplaced in vertical direction by means of an actuator 550.2, thusenabling the object 520 to be lifted (i.e., unloaded) from the supportsurface 510.1 or enabling the object 520 to be lowered (i.e., loaded)onto the support surface 510.1. In the arrangement as shown, theactuator 550.2 may e.g., be mounted to a positioning device 560configured to position the object table 600. In the arrangement asshown, the pin-shaped members 550.1 are considered to be made fromelectrically isolated materials. In the arrangement as shown, the objecttable 600 further comprises an electrical conductor 580 which connects atop surface of the pin shaped member 550.1 to the electrode 540. Inparticular, in the arrangement as shown, the electrical conductor 580comprises an electrically conductive wire having one end 580.1electrically connected to a top surface of the pin-shaped members 550.1and another end 580.2 connected to the electrode 540. By doing so, theelectrical conductor 580 is kept at the same potential as the electrode540. During the inspection process, the pin-shaped members 550.1 are ina retracted position such that there is no contact between thepin-shaped members 550.1 and the object 520. When the object needs to beunloaded, the voltage source 542 supplying the voltage of electrode 540will be turned off or will output a low voltage, e.g., 0V. In theexample as shown, the same voltage will be applied to the electricalconductor 580. When the object is subsequently unloaded, the pin-shapedmembers 550.1 will be moved upwards resulting in a contacting of the topsurface of the pin-shaped members 550.1 with the bottom surface 520.1 ofthe object 520. As a result, the object 520 will remain at the voltageas generated by the voltage source 542, and supplied to the electrode540, e.g., 0 V, during the unloading. By doing so, the risk of sparking,e.g., from the object 520 to the electrode 540, can be reduced.

The arrangement as schematically shown in FIG. 6 has been found tosuffer from the following drawbacks:

It has been observed that the application of the electrical conductor580 as schematically shown in FIG. 6 may result in the generation of acomparatively large electric field in the vicinity of the electricalconductor 580. As a result, an undesired so-called field emission orfield electron emission may occur. In particular when the appliedvoltage to the electrode 540 is comparatively high, such a risk occurs.At present, there is a tendency to increase the voltage as applied tothe electrode 540 during inspection, in order to improve the inspectionprocess.

As such, voltages in the range of −5 kV to −50 kV or higher may beapplied. When such voltages are applied to an electrode 540 as shown inFIG. 6, the application of the electrical conductor 580 in would resultin the mentioned field emission.

The known arrangement whereby an electrical connection 580 is providedbetween the pin-shaped member 550.1 of the loading/unloading mechanism550 and the electrode 540 may cause mechanical disturbances during theinspection process. As can be seen in FIG. 6, the electrical connection580 results in a permanent mechanical connection between the electrode540, which is mounted to the support member 510, and the pin shapedmember 550.1, which is mounted to the positioning device 560. Such amechanical short-circuit between the positioning device 560 and thesupport member 510 may cause a transmission of vibrations from thepositioning device 560 to the support member 510 supporting the object520. Such vibrations of the support member 510 may adversely affect theinspection process. For accurate positioning of the object, thepositioning device 560 may e.g., comprises a cascaded arrangement of afine positioning device and a coarse positioning device. The finepositioning device may also be referred to as a short stroke positioningdevice, whereas the coarse positioning device may also be referred to asa long stroke positioning device. In such an example, the support member510 may e.g., be accurately positioned by a short stroke positioningdevice (not shown) while the support member 510, together with the shortstroke positioning device can be displaced over comparatively largedistances by the positioning device 560. In such an example, 560 maye.g., refer to the mover of a linear or planar motor which is configuredto move the support member, together with the short stroke positioningdevice, over comparatively large distances.

One of the objects of the present disclosure is to overcome or at leastmitigate the aforementioned drawbacks of the arrangement as shown inFIG. 6.

In particular, in accordance with a first aspect of the presentdisclosure, measures are taken to avoid or mitigate the occurrence offield electron emissions and/or to avoid or mitigate the transmission ofvibrations towards the support member supporting the object that isinspected.

In accordance with the first aspect of the present disclosure, there istherefore provided, in some embodiments, an object table comprising:

-   -   a clamping mechanism for holding an object such as a substrate;    -   a loading/unloading mechanism configured to contact a bottom        surface of the object to load or unload the object;    -   and wherein the object table further comprises an electrical        conductor configured to electrically connect the object to a        predetermined voltage during at least part of an unloading        sequence of the object.

In some embodiments, the electrical conductor is configured to form alow mechanical stiffness connection when the object is held on theobject table. In some embodiments, such a low mechanical stiffnessconnection may be realised by appropriate shaping or forming theelectrical conductor. With respect to the meaning of a low mechanicalstiffness', it can be pointed out that, within the meaning of thepresent disclosure, a mechanical stiffness equal to zero is consideredan example of a low mechanical stiffness. In particular, in variousembodiments of the present disclosure, the electrical conductor can beconfigured to electrically connect the object to a predetermined voltageduring at least part of an unloading sequence of the object and may beopen, i.e. mechanically disconnected when the object is held on theobject table.

In some embodiments, the electrical conductor may e.g. have across-section and a mechanical stiffness of the electrical conductor islower than the mechanical stiffness of an electric wire having the samecross-section.

Such an example may e.g. be realised by providing the electricalconductor with a coil-shaped or spiral-shaped portion.

In some embodiments according to the first aspect of the presentdisclosure, the electrical conductor as applied comprises a coil shapedportion. An example of such an electrical conductor is schematicallyshown in FIGS. 7a and 7b . In the example as shown, an electricalconnector or wire 680 is connected to an electrode 640 during theinspection of an object 620. As such, the wire 680 may be at acomparatively high, e.g., negative, voltage during an inspectionprocess. In the example as shown, the electrical connector 680 isconnect at one end 680.1 to the top of a pin-shaped member 650.1 and atthe other end 680.2 to the electrode 640. In the example as shown, theelectrical wire 680 is advantageously arranged in a particular shape, inorder to mitigate the electric field that is generated when the wire isat the comparatively high voltage. In particular, the wire 680 comprisesa coil-shaped portion 680.3, i.e., a portion whereby the wire 680 isarranged in a spiraling manner As a result, the maximum electric fieldthat is generated by the wire when connected to a voltage source isreduced, thus reducing the risk of so-called field emission or fieldelectron emission. Within the meaning of the present disclosure,coil-shaped refers to having multiple windings or turns or beingarranged in a spiraling manner It may also be referred to as springshaped.

As an alternative to the use of an electrical wire as the electricalconductor, the use of a flexible PCB connector can be mentioned as well.Such a flexible PCB or flex PCB may be described as a sheet ofconductive material that is covered on both sides by an insulatinglayer. Such a flex PCB may easily be cut into a coil or spiral shape andapplied as flexible conductor in some embodiments of the presentdisclosure.

FIGS. 7a and 7b schematically show a possible arrangement of theelectrical conductor 680 as can be applied in some embodiments of thepresent disclosure. The electrical conductor 680 may e.g., be a barewire, i.e., an uninsulated wire, shaped as e.g., shown in FIGS. 7a, 7b .A part of the electrical conductor 680 may also be an electricallyinsulated or shielded cable that is arranged between the pin-shapedmember 650.1, in particular the top or tip thereof, and the electrode640 or a flex PCB. The electrical conductor 680 may also comprise afirst part comprising a bare, uninsulated wire and a second partcomprising an electrically insulated or shielded cable.

FIG. 7a schematically shows a cross-sectional view of a pin-shapedmember 650.1 and an electrical conductor 680 for two different positionsof the pin-shaped member. In the top portion of FIG. 7a , the pin-shapedmember 650.1 is in an elevated position, thereby contacting the object620, whereas in the bottom portion of FIG. 7a , the pin-shaped member650.1 is in a retracted position, such that the object 620 is arrangedon the object table 610. In the example as shown, the electrical wire680, i.e., the electrical wire forming the electrical conductor 680, isarranged in a spiraling manner around the pin-shaped member 650.1 and isarranged between the top of the pin-shaped member 650.1 and an electrode640. The electrode 640 may also be arranged on the object table 610,e.g., in a similar manner as electrode 540 is arranged on the supportmember 510. By arranging the electrical wire 680 in a spiraling, i.e.,coil-shaped manner, the electric field as generated around the conductorcan be mitigated.

FIG. 7b schematically shows a top view of a possible arrangement of anelectrical wire 680 between a top 690 of a pin-shaped member and anelectrode 640. In the example as shown, a first portion 680.3 of theelectrical wire 680 is arranged in a spiraling manner around the top, asecond portion 680.4 of the electrical wire 680 is arranged in ameandering manner towards the electrode 640.

In the embodiments as shown in FIGS. 6, 7 a and 7 b, the electricalconductor 680 is arranged to connect a top surface of a pin-shapedlifting member 650.1 to an electrode 640 of the object table 600. Aswill be appreciated by the skilled person, the electrical conductor 680may also be made between the top surface of the pin-shaped liftingmember 650.1 and an output terminal of a voltage source, e.g., a voltagesource such as voltage source 542. In such an example, the electricalconductor 680 may comprise an electrically conductive wire having oneend 680.1 connected to a top surface of the pin-shaped members 650.1 andanother end 680.2 connected to an output terminal such as outputterminal 542.1 of a voltage source 542. By doing so, the electricalconductor 680 is kept substantially at the same potential as theelectrode 640.

Regarding this alternative example, it can be pointed out that thevoltage source 542 may e.g., be arranged on the positioner 660. As such,the electrical conductor 680 would preferably at least partially be ashielded wire or cable extending between the output terminal 542.1 ofthe voltage source 542 and the pin-shaped member 650.1. in sucharrangement, it may be advantageous to have at least one unshieldedportion on the electrical connection, such unshielded portion having alower stiffness so as to mitigate the transfer of vibrations between thepositioner 660 and the object table 600.

As an alternative to providing the electrical conductor 680 with a coilshaped or spiraling portion, in order to mitigate or avoid fieldelectron emission, the application of an electrically conducting shieldmay be considered as well. Such an example is schematically shown inFIG. 7c . FIG. 7c schematically shows a cross-sectional view of apin-shaped member 650.1 and an electrical conductor 680, the pin-shapedmember 650.1 being in an elevated state, thereby lifting the object 620above the object table 610. In the example as shown, an electrical wire780 is arranged at one end 780.1 to the top of the pin-shaped member650.1 and, at another end 780.2 to an electrode 640. The electrode 640may e.g., be arranged on the object table 610, e.g., in a similar manneras electrode 540 is arranged on the support member 510. The example asshown further comprises an electric shield 790 that can e.g., be mountedto the object table 610. In the example as shown, the electric shieldmay e.g., have a semi-spherical shape and comprises a first aperture790.1 allowing the pin-shaped member to protrude and a second aperture790.2 allowing the electrical wire 780 to pass through. Such an electricshield 790 enables to mitigate or avoid field electron emission as well.

In the example as shown, the electrical wire 780 comprises a conductor780.3 connecting the wire to the shield 790, thus ensuring that theshield is at the same voltage as the wire it is shielding.Alternatively, the electrical wire 780 may comprises a first wireconnecting the electrode 640 to the shield 790 and a second wireconnecting the shield 790 to the pin-shaped member 650.1, in particularto a conductive top-portion of the pin-shaped member. Alternatively, theshield may be mechanically connected to the pin-shaped member 650.1 andmove along with the pin-shaped member. In such an example, theelectrical wire 780 may also comprise a first wire connecting theelectrode 640 to the shield 790 and a second wire connecting the shield790 to the pin-shaped member 650.1. alternatively, the shield may bemechanically connected to a conductive top portion of the pin-shapedmember 650.1. In such an example, only an electrical wire between theelectrode 640 and the shield 790 is required.

In the an example as described with reference to FIGS. 7a and 7b , theapplication of an electrical conductor having a coil shaped portion alsoprovides a reduced mechanical stiffness between the pin-shaped member650.1 and the electrode 640. By arranging at least part of theelectrical conductor 680 in a spiraling or meandering manner, a reducedmechanical stiffness of the conductor is obtained. As a result, atransmission of vibrations, e.g., from the pin-shaped member 650.1 tothe electrode 640, is reduced. This enables a more accurate inspectionprocess of the object 620.

In some embodiments of the present disclosure, the loading/unloadingmechanism comprises one or more lifting members, e.g., pin-shapedlifting members, of which a contact area that is configured to contactthe object during loading and unloading, is permanently grounded. Thiscan e.g., be realized, in some embodiments, by making the liftingmembers from an electrically conducting material. Alternatively, anelectrically conducting wire can be connected between the contact area,e.g., a top surface of the pin-shaped members, and an electric groundterminal.

It should be pointed out that in such an example, the pin-shaped liftingmembers may need to be retracted over comparatively large distances, inorder to avoid a sparking during the inspection process.

Depending on the applied voltage to the electrode 640 during theinspection process, it may be required to retract the pin-shaped liftingmembers, for example, several centimeters below the electrostatic clamp,e.g., 50 mm. In such an example, the permanently grounded pin-shapedmember should be retracted or lowered such that a distance between thepin-shaped member and the electrode 640, i.e., the high voltageelectrode of the object table, is large enough to avoid a dischargebetween the pin-shaped member and the electrode.

In an alternative example, the object table is provided with a dedicatedpin-shaped member for grounding the object during the unloading or atleast part of the unloading sequence of the object. Such an example isschematically shown in FIG. 8.

In FIG. 8, an object table 800 according to some embodiments of thepresent disclosure is schematically shown, the object table 800including, a support member 610 and an electrostatic clamp 630 thatprovides a support surface 610.1 for supporting an object 520, e.g., asemiconductor substrate. a support member 610 having a support surface610.1 for supporting an object 620, e.g., a semiconductor substrate orreticle. In the example as shown, an electrostatic clamp 630 is arrangedin a recess of the support member 610. The object table furthercomprises an electrode 640 and a loading/unloading mechanism 650.Further details such as the electrode arrangement of the electrostaticclamp 630 or the voltage supply for the electrode 640 are omitted forclarity.

In the example as shown, the object table 800 further comprises apin-shaped member 810 that is configured to contact the object 620, inparticular a bottom surface 620.1 of the object 620. In the example asshown, the pin-shaped member 810 is considered to be made from anelectrically conducting material and is permanently grounded. In theexample as shown, the pin-shaped member 810 can be moved in theindicated Z-direction by means of an actuator 815. In accordance withthe present disclosure, the permanently grounded pin-shaped member 810can be applied to, before and/or during an unloading sequence, or partthereof, ground the object 620, by raising the member 810 such that itcontacts the object 620, during the unloading. As a result of thegrounding, there will be no voltage induced in the object 620 such thatthe risk of sparking of the object 620, e.g., towards the electrode 640,is reduced. Also, in these embodiments, it is important to ensure thatthe pin-shaped member 810 is configured to be sufficiently retracted orlowered during the inspection, such that a distance between thepin-shaped member 810 and the electrode 640, i.e., the high voltageelectrode of the object table, is large enough to avoid a dischargebetween the pin-shaped member and the electrode.

The above-described embodiments whereby a permanently groundedpin-shaped member is applied provides the advantage that there is nowired electrical connection between the pin-shaped member and a highvoltage electrode of the object table which could result in atransmission of vibrations. It can also be pointed out that theapplication of such a dedicated permanently grounded pin-shaped membermay be comparatively straightforward since this member only needs tocontact the object during unloading. An accurate position control of themember, e.g., to control the position of the object duringloading/unloading, is therefore not required. In some embodiments of thepresent disclosure, whereby the object table is provided with a highvoltage electrode as described above, a discharging towards the highvoltage electrode is avoided or mitigated by increasing a distancebetween the high voltage electrode and the object that is to beunloaded, prior to the unloading sequence. This can e.g., be realized byconfiguring the high voltage electrode in such manner that it can beretracted, e.g., lowered or moved away from the object. In such anexample, the high voltage electrode may e.g., be configured as aretractable high voltage ring, which can e.g., be lowered prior to theunloading of the object. Alternatively, the clamping mechanism forholding an object of the object table can be configured to be moved awayfrom the high voltage electrode, prior to the unloading sequence. Insuch an example, the clamping mechanism holding the object may beelevated, relative to the high voltage electrode. As a result, thedistance between the clamping mechanism and the high voltage electrodeis increased, thus reducing the risk of a discharging from the object tothe high voltage electrode.

In some embodiments, the electrical connector as applied in the objecttable according to the present disclosure comprises two members. In suchan example, the two members of the electrical connector may beconfigured to form an electrical connection under certain conditions. Inparticular, the two members of the electrical connector may beconfigured to connect when the loading/unloading mechanism is in acertain position or in a certain range. By doing so, one can e.g.,ensure that a mechanical connection between the two members is absentduring an inspection process and is only present during at least a partof the unloading sequence. A transmission of vibrations via theelectrical connection during an inspection process may thus be avoided.FIGS. 9 and 10 schematically show two possible embodiments of such anarrangement. FIG. 9 schematically shows a pin-shaped member 900 as canbe applied in a loading/unloading mechanism as is applied in an objecttable according to the present disclosure. The pin-shaped member 900 isshown in two different positions relative to a support surface 910 ontowhich an object 920 can be supported. The pin-shaped member 900 asschematically shown has a top portion 900.1 that is electricallyconductive. The arrangement as shown further comprises an electricalconductor 950 comprising a first conductor member 950.1, e.g., aflexible, bendable, rod shaped electrical conductor that is hinged tothe top portion 900.1 of the pin-shaped member 900.1. The firstconductor member 950.1 is further configured such that it can pivot orrotate as indicated by the arrow 960, when the pin-shaped member 900 ismoved upwards. As a result of the pivoting or rotating, an end 950.11 ofthe first conductor member 950.1 can be made to contact a secondconductor member 950.2 of the electrical conductor 950. This isschematically shown in the upper part of the right part of FIG. 9. Inthe arrangement as shown, the end 950.11 is made to contact the secondconductor member 950.2 when the top portion 900.1 of the pin-shapedmember 900 touches the object 920. In the example as shown, the secondconductor member 950.2 may be connected to an electrical ground orground potential. When the pin-shaped member 900.1 is moved furtherupwards, as shown in the lower part of the right part of FIG. 9, thefirst conductor member 950.1 may bend, while remaining in contact withthe second conductor member 950.2. The first conductor member 950.1 canbe considered to operate as a cantilever. By suitable selection of theposition of the pivot point 960, one can arrange that a comparativelysmall displacement of the pin-shaped member 900 results in acomparatively large displacement of the end 950.11 of the firstconductor member 950.1 thereby ensuring that a sufficiently large gapexists between the end 950.11 and the grounded second conductor member950.2. Note that, instead of an electrical grounding of the secondconductor part or member 950.2, the second conductor member 950.2 mayalso be connected to an electrode such as the electrode 640 describedabove, or to the voltage source 642.

FIG. 10 schematically shows an alternative example whereby theelectrical connector is configured to provide a connection when theloading/unloading mechanism is in a particular position or range. In theexample as shown, the electrical conductor can also be considered tohave two members.

In the example as shown, a top portion 1000.1 of the pin-shaped member1000 is considered to be conductive, e.g., made of a conductive materialor comprising a conductive coating. Such conductive top portion can thusbe considered a first conductive member of an electrical conductor. Inthe example as shown, the pin-shaped member is configured to protrudethrough an aperture 1010 of a second conductive member 1020 of anelectrical connection or connector, whereby, when the pin-shaped member1000 protrudes the aperture 1010, an electrical connection is madebetween the pin-shaped member 1000 and the conductive member 1020, whichis, as schematically shown, connected to an electrode 640, which cane.g. be connected to a voltage source as discussed above. The conductivemember 1020 may also be directly connected to the voltage source. Inorder to ensure the electrical contact, the aperture may be providedwith a plurality of fine conductive elements 1030, e.g., brush or hairlike conductive wires which may be arranged to at least partly obscurethe aperture and which can deflect, as e.g., shown in the right portionof FIG. 10, when the pin-shaped member 1000 protrudes the aperture 1010.As such, during at least part of an unloading sequence of an object 920that is supported on a support surface 910, the object 920 iselectrically connected to a predetermined voltage or potential, i.e.,the potential applied to the electrode 640.

In the embodiments as shown, the second conductor members 950.2, 1020are connected to potential predefined voltage source, either directly orindirectly, e.g., via a high voltage electrode. These embodiments thusalso ensure that during at least part of an unloading sequence of theobject 920, the object 920 is connected to predefined voltage. By doingso, as indicated above, the voltage of the object can be controlled; avoltage increase of the object 920, e.g., caused by a surface charge ofan electrostatic clamp of the object table and a reducing capacitanceduring the unloading, can be avoided or mitigated.

In some embodiments of the present disclosure whereby the object tablecomprises an high voltage electrode as described above, a discharging tothe high voltage electrode, e.g. during an unloading of the object, canbe avoided or mitigated by applying an elevated voltage on the highvoltage electrode during said unloading or at least part of saidunloading.

In some embodiments, similar to the example as shown in FIG. 10, theelectrical connection as applied is configured in such manner that,during at least part of the unloading sequence of the object, acontrolled discharging of any build up charge on the object is realized.By appropriate shaping of at least one member of the electricalconnector, in particular by e.g., shaping it as a sharp needle, acontrolled discharge can occur between the two conductor members. Suchan example is schematically shown in FIGS. 11a and b.

FIG. 11a can be considered similar to FIG. 10, apart from the following:

In FIG. 11 a, the second electrical conductor member 1020, which isconnected to electrode 640, is provided with one or more needle shapedelectrical conductors 1110 arranged along the circumference of theaperture 1010, e.g., pointing inwards. Note that the needle shapedconductors need not obscure the passage of the pin-shaped member 1000through the aperture 1010. When the pin-shaped member 1000 protrudes theaperture 1010, e.g., to unload the object 920 from the support surface910, a voltage increase of the object can be prevented since theelectrical connection or connector, formed by the conductive top portion1000.1 and the second electrical conductor member 1020 provided with theneedle shaped conductors 1110, will enable a discharging 1120 of theobject, during at least part of the unloading sequence.

Due to the controlled discharge 1120, the object's voltage will nolonger rise during the unloading stage; the risk of a sparking ordischarging towards the object table, e.g., towards the electrode on theobject table is therefore mitigated.

As will be understood, alternative arrangements whereby a conductiveportion of the pin-shaped member is configured to make an electricalconnection in a certain positional range and is disconnected in anotherrange can be devised as well.

In the example as shown, the needle shaped conductors 1110 are providedon the second electrical conductor member 1020. Alternatively, they mayalso be provided on the conductive top portion 1000.1 of the pin-shapedmember 1000 as well. In such an example, the second electrical conductormember 1020 may be suitably shaped, in order to form the electricalconnection is a required positional range. Such an example isschematically shown in FIG. 11 b. In the example as shown, theconductive top portion 1000.1 of the pin-shaped member 1000 is providedwith needle shaped conductors 1110. When the pin-shaped member iselevated, as can be seen on the right of FIG. 11b , the needle shapedconductors 1110 may interact with the second electrical conductor member1020 so as to allow a discharge. As will be understood, the embodimentsof FIGS. 11a and 11b may be combined as well thus having needle shapedconductors 1110 on both the second electrical conductor member 1020 andthe conductive top portion 1000.1 of the pin-shaped member 1000.

FIG. 12 e.g., shows an arrangement whereby a first conductor member 1210of a pin-shaped member 1200 of a loading/unloading mechanism isconfigured to contact a second conductor member 1220 having a flexibleconductive member 1220.1. When the pin-shaped member 1200 is lifted,e.g., to unload an object 920 from a support surface 910, as shown inthe right portion of FIG. 12, the first conductor member 1210 cancontact the second conductor member 1220, thereby establishing a contactbetween the object 920 and a predefined voltage, e.g., via a connectionto an electrode 640, during at least part of the unloading sequence ofthe object 920.

Yet an alternative manner to switch the pin-shaped member between aconnected state, whereby the top surface of pin-shaped member is e.g.,grounded or connected to an HV source and a disconnected state, wherebythe top surface is e.g., allowed to adjust its potential, i.e., have afloating potential, is schematically shown in FIG. 13.

In FIG. 13, an object table 1300 according to some embodiments of thepresent disclosure is schematically shown, the object table 800including a support member 610 and an electrostatic clamp 630 thatprovides a support surface 610.1 for supporting an object 520, e.g., asemiconductor substrate or reticle. In the example as shown, theelectrostatic clamp 630 is arranged in a recess of the support member610. The object table further comprises an electrode 640 and aloading/unloading mechanism 1350 comprising pin-shaped members 1350.1and 1350.2. Further details such as the electrode arrangement of theelectrostatic clamp 630 or the voltage supply for the electrode 640 areomitted for clarity.

In FIG. 13, pin-shaped member 1350.1 may e.g., be a conventionalisolated pin-shaped member as e.g., applied in a known loading/unloadingarrangement. The pin-shaped member 1350.2 however comprises a conductivetop-portion 1350.21, a conductive bottom portion 1350.22, e.g.,configured to be grounded, and a middle portion 1350.23 which can befilled with a gas that can be ionized. The middle portion 1350.23 maye.g., be a tube provided with a pair of electrodes for ionizing the gas.In such an arrangement, the conductivity of the pin-shaped member 1350.2can be controlled; by controlling the voltage supplied to the pair ofelectrodes, an ionized gas can be generated in the middle portion1350.23, thereby rendering the middle portion conductive. By doing so,the top portion 1350.21 becomes electrically grounded to the bottomportion 1350.22. In such an example, the pin-shaped member 1350.1 maythus be made conductive when an object is to be loaded or unloaded andmay be made to isolate when the object is being inspected.

As an alternative to ionizing the gas that is inside the middle portion,one or more of the pin-shaped members may be configured to receive a gasthat is already ionized. In such an example, the conductivity of the oneor more pin-shaped members may be controlled by supplying and evacuatingalready ionized gas to and from the pin-shaped member, rather thanionizing a gas that is inside the pin-shaped member.

Suitable gases that may be comparatively easily ionized may e.g.,include Argon or Neon.

In the embodiments as discussed so far, the object is configured to beconnected to a predetermined voltage or voltage source during at leastpart of the unloading stage via the loading/unloading mechanism, inparticular via a pin-shaped member of said mechanism. Alternativearrangements of the electrical connector may however be devised as well.

As an example, the electrical connection between the object and thepredetermined voltage may be realized by contacting the edge or even thetop surface of the object, e.g., using a movable electrode or electricalconnector. A gripper of a handler or handling robot may be suited forthis. As such, in some embodiments, the object table according to thepresent disclosure can be configured to e.g. co-operate with a robothandler, such a robot handler e.g. comprising and end effector gripper,e.g. a conductive or semi-conductive end effector gripper, which isconfigured to contact the object that is to be unloaded, e.g. an edge ora top surface of the object, during at least part of the unloadingsequence of the object. When the end effector gripper is grounded, suchan arrangement also enables to ensure the, at least during part of theunloading sequence, the object is connected to a predefined voltage orvoltage source. Alternative to the application of the end effectorgripper to contact the object during at least part of the unloadingsequence, the end effector gripper of the robot handler may comprise anelectrode or contact, e.g., a thin flexible conducting wire, forcontacting the object during at least part of the unloading. In someembodiments, a high voltage electrode of the object table may beelectrically isolated from the object by a semi-conductive end effectorgripper inserted in between the object and the high voltage electrodeduring such an unloading or during at least part of such unloading.

Alternatively, a flexible conductive contact can be provided, e.g.,along the edge of the object, such that, during at least part of theunloading sequence of the object. Such an example is schematically shownin FIG. 14. The object table 1400 as schematically shown in FIG. 14comprises, a support member 610 and an electrostatic clamp 630 thatprovides a support surface 610.1 for supporting an object 520, e.g., asemiconductor substrate. a support member 610 having a support surface610.1 for supporting an object 620, e.g., a semiconductor substrate orreticle. In the example as shown, the electrostatic clamp 630 isarranged in a recess of the support member 610. The object table furthercomprises an electrode 640 and a loading/unloading mechanism 1450comprising pin-shaped members 1450.1. In some embodiments, theloading/unloading mechanism 1450 may e.g., comprise three pin-shapedmembers. Further details such as the electrode arrangement of theelectrostatic clamp 630 or the voltage supply for the electrode 640 areomitted for clarity. In the example as shown, the object table 1400further comprises a flexible electrical connector or contact 1460 forconnecting to an edge or bottom surface of the object 620, during atleast part of an unloading sequence of the object 620. As shown, theflexible electrical connector 1460 is connected the electrode 640, e.g.,via an electrical wire 1460.1. The flexible electrical connector 1460can e.g., be configured to contact the object in a sliding mannerAlternatively, a roller contact can be provided at the end portion1460.2 of the flexible electrical connector 1460, thereby mitigating therisk of generating particles. The electrical connector 1460 can e.g., bea leaf-spring or leaf-spring like member that can bend downward when theobject is loaded and can bend upward during at least part of anunloading, so as to remain in contact with the object 620.

As an alternative to the arrangement as shown in FIG. 14, the flexibleelectrical connector 1460 may e.g., be a spring-shaped connector that ismounted underneath the bottom surface 620.1 of the object, i.e., betweenthe support surface 610.1 and the bottom surface 620. 1 of the object.Said spring may than be connected to the high voltage electrode 640, ina similar manner as the connector 1460 is connected to the electrode.

As an alternative to the electrical connector 1460 as shown in FIG. 14,the electrical connector as applied may comprise an electricallyconductive spring or spring like conductor that is arranged between thesupport surface 610.1 and the bottom surface 620.1 of the object that issupported. Such a spring or spring like conductor can be configured tobe compressed when the object 620 is loaded and configured to expandduring at least part of an unloading sequence of the object, so as toremain in contact with the object 620. It may further be connected tothe electrode 640 via an electrical wire.

Yet an alternative manner to provide in an electrical connection betweenthe object and a predetermined voltage, e.g., an electric groundpotential, is by purging the volume between the object and theelectrostatic clamp with an ionized gas. Such a gas can be considered toform an electrical connection between the object and any material in thevicinity, e.g., an electrode surrounding the object such as electrode640 shown above. As such, an ionized gas may, within the meaning of thepresent disclosure, be considered to an electrical conductor as well,i.e., a gaseous electrical conductor. It can further be pointed out thatthe application of such an ionized gas flow may also, at least partly,result in a cancelling or compensation of the surface charge that wasgenerated on the surface of the electrostatic clamp 630.

As such, in some embodiments of the present disclosure, a purging of thevolume underneath the object and/or above the electrostatic clamp usingan ionized gas is performed, e.g., periodically, in order to mitigate oravoid the build-up of a surface charge on a surface of the electrostaticclamp 630.

In such an example, rather that attempting to mitigate any adverseeffects of the build-up surface charge, e.g., by grounding the objectduring the unloading, the surface charge itself is mitigated or removed.

According to a second aspect of the present disclosure, there isprovided an object table configured to hold an object such as asubstrate, the object table comprising:

-   -   an electrostatic clamp configured to hold the object;    -   a measurement unit configured to determine an electric        characteristic of the electrostatic clamp, the electric        characteristic being representative of a charge state of the        electrostatic clamp;    -   a control unit configured to control, during an unloading of the        object, a power supply of the electrostatic clamp, based on the        determined electric characteristic.

According to the second aspect of the present disclosure, there isprovided an object table that enables to control, before and/or duringan unloading of the object a power supply of the electrostatic clamp ofthe object table, based on a determined electric characteristic of theelectrostatic clamp, whereby the electric characteristic represents acharge state of the electrostatic clamp, especially the residual chargeon the electrostatic clamp when no voltage is applied to theelectrostatic clamp.

As explained above, with reference to FIG. 5, the use of anelectrostatic clamp for holding an object such as a semiconductorsubstrate on an object table, may result in the occurrence of a surfacecharge on a surface of the electrostatic clamp. This surface chargecauses an attractive force, also referred to as a sticking force,between the object and the object table, in particular between theobject and the electrostatic clamp of the object table. As will beunderstood by the skilled person, this sticking force has to beovercome, in order to unload the object from the object table. Phraseddifferently, an unloading of the object from the object table willrequire a force to be exerted on the object that is at least equal toand directed in opposite direction to the sticking force. So, the largerthe sticking force, the larger the unloading force, i.e., the forceneeded to unload the object, will need to be. As the unloading force istypically performed using a loading/unloading mechanism comprising oneor more pin-shaped members, the unloading force that can be generatedmay be comparatively small. Also, the application of a comparativelylarge unloading force via the one or more pin-shaped members onto theobject may cause damage to the object.

In a typical object table, as e.g., described with reference to FIG. 5,an electrostatic clamp as applied in the object table would not bepowered during the unloading of the object. In accordance with thesecond aspect of the present disclosure however, it is proposed tocontrol the power supply of the electrostatic clamp, i.e., the power assupplied to the electrostatic clamp, during the unloading, based on adetermined electric characteristic which represent a charge state of theelectrostatic clamp. By doing so, as will be explained in more detailbelow, account can be taken of the charge stage of the electrostaticclamp and appropriate measures can be taken to compensate, counteract ormitigate the effect of the charge state on clamping force, i.e., theresidual or permanent clamping force.

As an alternative, or in addition, to controlling the power supply ofthe electrostatic clamp during the unloading, the power supply of theelectrostatic clamp may also be controlled during the clamping of theobject, based on the determined electric characteristic. In particular,in some embodiments of the present disclosure, the charge state of theelectrostatic clamp or the effect of the charge state on the clampingforce may be taken into account during the clamping of the object: byappropriately powering the electrostatic clamp, the effect of themeasured or determined charge state of the electrostatic clamp can betaken into account thus ensuring that a required clamping force isobtained. As an example, when the charge on the clamp surface near anegative electrode of the clamp is negative, a smaller clamping voltageis required to result in the same net clamping force. Similarly, anegative charge near a positive clamping electrode would require alarger positive voltage on the positive clamping electrode to achievethe same clamping force, since the surface charge counteracts theclamping force there.

In accordance with the second aspect of the present disclosure, thepower supply of the electrostatic clamp, or the power supplied to theelectrostatic clamp can be controlled, based on the determined electriccharacteristic, in such manner that the sticking force at least partlycan be compensated, counteracted or mitigated. Further, by doing so, adischarging between the object and the object table, e.g., a highvoltage electrode on the object table, may be avoided as well.

FIG. 15 schematically shows an example of an object table 1500 accordingto the second aspect of the present disclosure. In FIG. 15, an objecttable 1500 according to some embodiments of the present disclosure isschematically shown, the object table 1500 including, similar to theobject table 600 of FIG. 6, a support member 610 having a supportsurface 610.1 for supporting an object 620, e.g., a semiconductorsubstrate. In the example as shown, an electrostatic clamp 630 isarranged underneath the support surface 610.1, e.g., embedded in thesupport member 610. In the example as shown, the object table furthercomprises, as optional features, an electrode 640 and aloading/unloading mechanism 650. Further details such as the electrodearrangement of the electrostatic clamp 630 or the voltage supply for theelectrode 640 are omitted for clarity. In accordance with the secondaspect of the present disclosure, the object table 1500 furthercomprises a measurement unit 1510 configured to determine an electriccharacteristic of the electrostatic clamp, indicated by the dotted line1510.1, the electric characteristic being representative of a chargestate of the electrostatic clamp 630. In accordance with the secondaspect of the present disclosure, the object table 1500 furthercomprises a control unit 1520 configured to control, during an unloadingof the object 620, a power supply 1530 of the electrostatic clamp 630,based on the determined electric characteristic. Line 1530.1 indicatesthe powering of the electrostatic clamp 630 by the power supply 1530.

The power supply 1530 can e.g., power the electrostatic clamp 630 byproviding an appropriate voltage to the one or more electrodes of theelectrostatic clamp 630. The power supply 1530 can e.g., be controlledby a control unit 1520 of the object table 1500. In accordance with thesecond aspect of the present disclosure, the control unit 1520 isconfigured to control, during an unloading of the object 620, the powersupply 1530 of the electrostatic clamp 630, indicated by the arrow1520.1, based on the determined electric characteristic, which can e.g.,be provided by means of an input signal 1520.2 to the control unit 1520.

In accordance with the second aspect of the present disclosure, themeasurement unit 1510 of the object table 1500 is configured todetermine an electric characteristic which is representative of a chargestate of the electrostatic clamp. The charge state of the electrostaticclamp can e.g., be indicative for a surface charge that has been buildup on a surface of the electrostatic clamp.

In accordance with the present disclosure, various methods are devisedto determine the electric characteristic and to control the power supplyof the electrostatic clamp based on the determined electriccharacteristic.

A first manner to determine the electric characteristic of theelectrostatic clamp is to perform a measurement during an initialportion of an unloading sequence of the object, e.g., a portion of theunloading sequence which starts when the object is still on the supportsurface.

In some embodiments, the measurement unit 1510 is configured todetermine, during said initial portion, the electric characteristic ofthe electrostatic clamp representing a charge state of the clamp byperforming a measurement. Said measurement can e.g., be a measurement ofa characteristic of the electrostatic clamp or a measurement of acharacteristic of the object that is being unloaded.

In the former case, i.e., where a measurement of a characteristic of theelectrostatic clamp is performed, the measurement unit 1510 may e.g., beconfigured to measure a current from or towards the one or moreelectrodes of the clamp as the electric characteristic. In such case,the power supply 1530 may e.g., be configured to maintain theelectrodes, during the initial portion of the unloading sequence, at apredetermined potential, e.g., zero volt, i.e., whereby the power supplyprovides a voltage of 0 V to the electrodes.

However, in order to maintain the voltage at 0 V during the unloading, acurrent will flow to or from the electrodes, due to the presence of thecharges on the electrostatic clamp. This can be understood as follows:when the object is unloaded, i.e., the distance between the object andthe clamp is increased, a voltage will be induced in the object (asexplained above) due to the changed capacitance formed by the object andthe clamp and the surface charge on the electrostatic clamp. This changein capacitance also affects the electrodes of the electrostatic clamp inthat, due to the presence of the surface charge, the voltage on theelectrodes would also change, in case it was not imposed by the powersupply. As such, in order to remain at 0 V, there will be a current fromor towards the electrodes, during the unloading sequence, when the powersupply 1530 of the electrostatic clamp 630 holds the clamp at 0 V. Asthis current occurs due to the presence of the surface charge on theclamp, it can be considered to be representative of the charge state ofthe electrostatic clamp or the surface charge on the clamp. Aftermeasurement of the current as the electric characteristic representingthe charge state, the control unit 1520 may then control the powersupply 1530 of the electrostatic clamp 630 based on the electriccharacteristic. In particular, the control unit 1520 can control thepower supply in such manner that a voltage is provided to the one ormore electrodes which counteracts or mitigates the effect of the chargestate of the electrostatic clamp.

The required voltage for mitigating the effect of the charge state ofthe electrostatic clamp can e.g., be determined based on empirical data,e.g., experimental data, e.g., combined with simulations.

In another example, the measurement unit can be configured to measure,as an electric characteristic of the electrostatic clamp representingthe charge state of the clamp, a voltage of the one or more electrodesof the clamp, e.g., during the unloading sequence or a part thereof. Insuch an example, the power supply 1530 may e.g., be configured todisconnect the one or more electrodes of the electrostatic clamp fromthe power supply during the unloading sequence or a part thereof. Bydoing so, the potential or voltage at the one or more electrodes becomesfloating or undefined. As a result, the potential or voltage of theelectrodes can vary during the unloading sequence or a part thereof.When a surface charge is present on the electrostatic clamp, the voltageat the one or more electrodes will indeed vary when the object isunloaded, due to the changed capacitance. As such, in these embodiments,the measurement unit 1510 may be configured to measure a voltage at theone or more electrodes of the electrostatic clamp as an electriccharacteristic representing a charge state of the electrostatic clamp,during the unloading sequence of the object. Based on this electriccharacteristic, the control unit 1520 controlling the power supply 1530may then, control the power supply of the electrostatic clamp 630. Inparticular, the control unit 1520 may control the power supply 1530 insuch manner that a voltage is provided to the one or more electrodeswhich counteracts or mitigates the effect of the charge state of theelectrostatic clamp.

In yet another example, as mentioned above, the measurement unit 1510 isconfigured to determine the electric characteristic representing acharge state of the electrostatic clamp by performing a measurement onthe object, rather than on the electrostatic clamp. As explained abovewith respect to the first aspect of the present disclosure, when anobject is unloaded from an electrostatic clamp having a surface charge,a voltage will be induced (assuming that the object is not grounded) insaid voltage, due to the surface charge and due to the change incapacitance when the object is moved away from the clamp. As such, insome embodiments, the measurement unit as applied in an object tableaccording to the present disclosure can be configured to determine,during an initial portion of an unloading sequence, measure a voltageinduced in the object. As the induced voltage is directly related to thesurface charge or charge state of the electrostatic clamp 630, it can beused to determine an electric characteristic of the electrostatic clamp,the electrical characteristic representing a charge state of the clamp.Once the electric characteristic is determined, it can be applied in asimilar manner as discussed above, to control the power supply 1530which powers the electrostatic clamp 630. In order to measure thevoltage induced in the object during the initial portion of theunloading sequence, electrical connectors or conductors as describedabove may e.g., be applied. In particular, use can e.g., be made of aconductive pin-shaped member of a loading/unloading mechanism to serveas a probe to measure an induced voltage in the object. As such, basedon the measured voltage induced in the object during an initial portionof the unloading sequence, the control unit can control the power supplypowering the electrostatic clamp in such manner that a voltage isprovided to the one or more electrodes which counteracts or mitigatesthe effect of the charge state of the electrostatic clamp.

In yet another example, the measurement unit 1510 is configured todetermine the electric characteristic representing a charge state of theelectrostatic clamp by performing a current measurement on the object.In case the object is connected, during the unload sequence, to apredetermined potential, e.g., an electric ground potential, a currentwill flow to or from the object when the object is unloaded, in asimilar manner as a current will flow to or from the electrodes of theelectrostatic clamp when these electrodes are connected to azero-voltage source during the unloading. Since this current is directlyrelated to the surface charge or charge state of the electrostatic clamp630, it can be used to determine an electric characteristic of theelectrostatic clamp, the electrical characteristic representing a chargestate of the clamp. Once the electric characteristic is determined, itcan be applied in a similar manner as discussed above, to control thepower supply 1530 which powers the electrostatic clamp 630. In order tomeasure the current that flows to or from the object during the initialportion of the unloading sequence, electrical connectors or conductorsas described above may e.g., be applied. As described above, suchelectrical connectors or conductors are configured to keep contact withthe object, at least during part of the unloading sequence thus enablingthe monitoring/measuring of the voltage of the object. As such, based onthe measured voltage induced in the object during an initial portion ofthe unloading sequence, the control unit can control the power supplypowering the electrostatic clamp in such manner that a voltage isprovided to the one or more electrodes which counteracts or mitigatesthe effect of the charge state of the electrostatic clamp.

In the embodiments of the object table according to the second aspect ofthe present disclosure as described so far, the object table, inparticular the measurement unit and the control unit of the objecttable, is configured to, when unloading a particular object:

-   -   Determine an electric characteristic of the electrostatic clamp        by performing a measurement during an initial portion of the        unloading sequence, and    -   Controlling the power supply powering the electrostatic clamp        before and/or during at least part of the unloading sequence        based on the electric characteristic.

In the embodiments as described, the electric characteristic was of theelectrostatic clamp was determined based on a current or voltagemeasurement of the electrostatic clamp or the object, during theunloading sequence or a portion thereof.

Such a determination can e.g., be performed during a portion or part ofthe unloading sequence, e.g., an initial portion of an unloadingsequence of an object. When the electrical characteristic of the clampis determined during an initial portion of the unloading, thisinformation may immediately be applied to control the power supplypowering the clamp during a next of following portion of the unloadingsequence. In such an example, the control of the power supply duringunloading of an object is thus based on measurements performed during aninitial part of the unloading sequence of said object. Such arrangementmay require a comparatively fast processing of the measurements and/orcomparatively fast control of the power supply. The following describesan alternative approach.

It is important to point out that the electrical characteristic of theelectrostatic clamp which represent a charge state of the electrostaticclamp may also be determined in other ways. In this respect, it is worthmentioning that the surface charge of the electrostatic clamp does notchange rapidly, rather, it changes gradually during the processing ofmultiple objects. When considering this, it can be realized that one canalso determine an electric characteristic of the electrostatic clamp atother instants than during an initial portion of an unloading sequenceof an object. In particular, the electric characteristic representing acharge state of the electrostatic clamp may also be determined earlierthan during an initial portion of the unloading sequence of the object.

In particular, the electric characteristic representing a charge stateof the electrostatic clamp may also be determined during a loading ofthe object. During a loading of the object, the object may e.g., belowered onto the electrostatic clamp, e.g., by a loading/unloadingmechanism as described above, during which a voltage may be induced inthe object, or a current may flow from or towards the object, dependingon whether the object is isolated or grounded. Such induced voltage oroccurring current may also be used to determine the electriccharacteristic of the electrostatic clamp, since it is directly relatedto or caused by the charge state of the electrostatic clamp. As such, insome embodiments, of the present disclosure, the object table may beconfigured to:

-   -   Determine an electric characteristic of the electrostatic clamp        by performing a measurement during a loading sequence of an        object on the electrostatic clamp, and    -   Controlling the power supply powering the electrostatic clamp        before and/or during at least part of the unloading sequence of        the object, based on the electric characteristic.

Alternatively, the electrical characteristic representing a charge stateof the electrostatic clamp may be determined during a loading orunloading of a previous object. In such an example, the charge state asdetermined during the loading or unloading of a first object is used tocontrol the powering of the electrostatic clamp during at least part ofthe unloading sequence of a second, subsequent, object. Such an exampleprovides the advantage that there is more time available to determinethe charge state and to determine the required power control of theelectrostatic clamp.

Since it has been observed that the charge state of the electrostaticclamp only gradually changes over time, one does not even have todetermine the electric characteristic of the electrostatic clamp forevery object that is to be unloaded. Phrased differently, it may besufficient to determine the electric characteristic representing thecharge state of the electrostatic clamp once per n objects that areprocessed, n e.g., being equal to 5 or 10. In some embodiments, thenumber n may e.g., be determined based on an identified charge state ofthe electrostatic clamp or a history of the identified charged stage. Asan example, when the identified charge state is large or when the changeof the charge states in a consecutive measurement is large, a smallernumber n can be selected. N may however be as small as 1.

In such an example, the electric characteristic of the electrostaticclamp may thus be determined during the loading and/or unloading of afirst object, whereas the electric characteristic as determined is usedto control the power supply during the unloading of a second object,different from the first object. As such, in some embodiments, theobject table according to the second aspect of the present disclosurecan be configured to:

-   -   Determine an electric characteristic of the electrostatic clamp        by performing a measurement during a loading and/or unloading        sequence of a first object on or resp. off the electrostatic        clamp, and    -   Controlling the power supply powering the electrostatic clamp        before and/or during at least part of the unloading sequence of        a second object that is different from the first object, based        on the electric characteristic.

The charge state distribution of the electrostatic clamp across all, orpart, of the clamp surface may be inhomogeneous. That is to say, thecharge state may be unevenly distributed over a region of the clampsurface that corresponds to clamping electrodes of the electrostaticclamp. For example, across the region there may be a combination ofnegative and positive charges, or multiple equally-signed but differentmagnitude charges.

A clamping voltage may be applied to these clamping electrodes togenerate a repelling force or to neutralize the charge on the clampsurface. For each electrode, the applied voltage may be determineddependent on the net charge of the clamp surface. However, such a singlevoltage for these clamping electrodes may not be appropriate or notsufficient in the situation of the charge distribution. Residual forcedistribution, being uneven across all or part of the clamp surfacebecause the single determined voltage may be too low, or too high, forsome parts of the clamp surface when, across the clamp surface, theremay be a combination of negative and positive surface charges ormultiple equally-signed but different magnitude surface charges aroundthese clamping electrodes.

In yet another example, the above-identified problem may be avoided byarranging for the charge distribution to be evenly distributed over theall or part of the lamp surface. For example, a semi-conductive coatingmay be applied to regions of the clamp surface. The semi-conductivecoating should be arranged so that it does not electrically connectregions of the clamp surface corresponding to different clampingelectrodes such that some of these clamping electrodes electrically arenot electrically connected and form electric short circuit.

In yet another example, the above-identified problem is solved byproviding two or more electrodes and separately controlling the voltagesapplied to each electrode. For example, instead of having a singlepositive electrode and a single negative electrode, the present exampleincludes there being two or more positive electrodes and/or two or morenegative electrodes. Separate voltages may be applied to each electrodeas appropriate for overcoming an uneven charge distribution of theregion of the clamp surface around the electrode. Each applied voltageto an individual electrode may be determined according to at least someof the techniques for determining a voltage to be applied at anelectrode according to the second aspect of the present disclosure. Eachelectrode may also be a clamping electrode for applying a holding forceto, or releasing, an object. Alternatively, one or more of theelectrodes may be used to only apply a force for counteracting theresidual force caused by the local charge distribution at said one ormore electrodes with said one or more electrodes not being used to applya holding force to an object.

Specific orientations have been given when describing the relativearrangement of components. It will be appreciated that theseorientations are given purely as examples and are not intended to belimiting. For example, the xy-stage of the positioning device 180 hasbeen described as being operable to position an object in asubstantially horizontal plane. The xy-stage of the positioning device180 may alternatively be operable to position an object in a verticalplane or in an oblique plane. Orientations of components may vary fromthe orientations described herein whilst maintaining their intendedfunctional effect of said components.

FIG. 16 schematically depicts an example of an object table 1600according to the third aspect of the present disclosure. The objecttable 1600 may substantially have the same structure as the objecttables described above, except for the following. The object table 1600is for holding an object 620. The object table 1600 comprises anelectrostatic clamp 630, an ionizer device 1610, a control unit 1520 anda measurement unit, e.g. including a force sensor 1620 and/or a gapsensor 1630 and/or a height sensor 1640. The electrostatic clamp 630 isfor clamping the object 620 on the object table 610. The measurementunit is configured to provide a force signal 1520.3 representative of aresidual force applied by the electrostatic clamp 630 on the object 620while unloading the object 620 from the electrostatic clamp 630. Theionizer device 1610 is configured to provide an ionized flow of gas1610.1 to the electrostatic clamp 630, working as a neutralizer toneutralize the residual charge on the electrostatic clamp 630. Thecontrol unit 1520 is arranged to control the ionizer device 1610 basedon the force signal 1520.3.

In an alternative example, the object table 1600 may comprise theelectrostatic clamp 630, the ionizer device 1610 and the control unit1520. The electrostatic clamp is for clamping the object 620 on theobject table 610. The ionizer device 1610 is for providing an ionizedflow of gas. The control unit 1520 is arranged to control the ionizerdevice 1610 to provide the ionized flow of gas to the electrostaticclamp.

The control unit 1520 may be arranged to receive an information signalrepresentative of a residual force or a residual charge. The residualforce is applied by the electrostatic clamp 630 on the object 620 duringunloading of the object 620 from the electrostatic clamp 630. Theresidual charge is an electrostatic charge present on the electrostaticclamp 630 when no charge voltage is applied to the electrostatic clamp630. The control unit 1520 may be arranged to control the ionizer device1610 based on the information signal. Ideally, there is no residualforce and no residual charge during unloading of the object 620 from theelectrostatic clamp 630. However, due to charge build-up on theelectrostatic clamp 630, a residual force and/or a residual chargeremain, even when no charge voltage is applied to the electrostaticclamp 630.

The information signal may comprise measurement information. Asindicated below, a measurement unit may provide measurement informationin the form of a measurement signal. More examples of measurementinformation are given below and may include information about the amountof force needed to lift the object 620 from the object table 610,orinformation about a gap or a capacitance between the object 620 and theobject table 610. Other measurement information may represent a shape ofthe object 620 and/or the amount of power needed to unclamp and unloadthe object 620 from the object table 610. The information signal may bean internal signal of the electrostatic clamp 630 and/or anloading/unloading mechanism representing the residual force and/or theresidual charge such as a signal representing a detection that theloading/unloading mechanism is not able to lift the object 620 from theelectrostatic clamp 630.

The information signal may comprise estimated information. Whereasmeasurement information is based on measurements representative of theresidual force or residual charge, the estimated information is based onother parameters, such as empiric data and simulations. Estimatedinformation may include that after a certain amount of time or afterunloading a certain amount of objects 620, the control unit 1520 needsto control the ionizer device 1610 and/or the discharge voltage tocounter the build-up residual charge or residual force. Simulations maypredict after how much time or how many times unloading an object 620the residual charge and the residual force are at the edge of anacceptable level. The estimated information may include informationobtained from outside the object table 1600, for example via an externalmeasurement device measuring the residual charge of the object 620outside the object table 1600, for example in a further processing stepof the object 620. The estimated information may include the estimatedresidual charge according to embodiments of the present disclosure,particularly the embodiments of the second aspect and the fifth andsixth aspect of the present disclosure. The control unit 1520 may makeuse of measurement information or estimated information or a combinationof measurement information and estimated information.

The object table 1600 may further comprise a measurement unit. Themeasurement unit is configured to provide a measurement signalrepresentative of the residual force or the residual charge. Theinformation signal comprises the measurement signal. The control unit1520 is arranged to control the ionizer device 1610 based on themeasurement signal.

In some embodiments, there is provided the object table 1600, comprisingthe electrostatic clamp 630 and the control unit 1520. The electrostaticclamp 630 is for clamping the object 620 on the object table 610. Thecontrol unit 1520 is arranged to provide to the electrostatic clamp 630a charge voltage to clamp the object 620 on the electrostatic clamp 630and a discharge voltage to unclamp the object 620 from the electrostaticclamp 630. The discharge voltage may unclamp the object 620 by reducingthe clamping force to, for example, substantially zero, or by providinga repulsing force to push the object 620 away from the electrostaticclamp 630. The control unit 1520 is arranged to receive the informationsignal representative of a residual force or a residual charge. Thecontrol unit 1520 is arranged to provide the discharge voltage by apower source, working as a neutralizer to neutralize the residual chargeon the electrostatic clamp 630, based on the information signal.

The discharge voltage may have a polarity opposite to the chargevoltage. In some embodiments, the object table 1600 may also comprise anionizer device such as the ionizer device 1610 for providing the ionizedflow of gas 1610.1 to the electrostatic clamp 630. The control unit 1520is arranged to control the ionizer device 1610 based on the informationsignal.

After using the electrostatic clamp 630 to clamp the object 620, aresidual charge may remain on the electrostatic clamp 630 even thoughthe charge voltage is no longer applied.

Ideally, there would be no residual charge and thus no force applied bythe electrostatic clamp 630 on the object 620 when the charge voltage isno longer applied. A residual force is created by the residual chargeand by gravity. Gravity adds to the residual force by the weight of theobject 620 that is supported by the electrostatic clamp 630. Thecontribution to the residual force by the residual charge may be muchbigger than the contribution of gravity, for example 10 times or 100times or more. The residual force keeps the object 620 clamped on theelectrostatic clamp 630. To unload the object 620 from the electrostaticclamp 630, an unloading force needs to be applied to the object 620 thatexceeds the residual force. However, the higher the unloading force, themore particles are generated during unloading the object 620. Theparticles may pollute the object 620 or the environment in which theobject 620 is processed. After repeatedly loading and unloading objects620 on and from the electrostatic clamp 630, the residual force maybecome so large that at a certain moment it is no longer possible tounload the object 620 from the electrostatic clamp 630 without extrememeasures. An extreme measure may be unloading the object 620 from theelectrostatic clamp 630 manually.

The object table 1600 as schematically shown in FIG. 16 enables toremove, alleviate or mitigate the necessity of such measures, by usingan ionizer device 1610, by applying a discharge voltage, or acombination thereof. When the measurement unit 1620 provides themeasurement signal, the control unit 1520 is arranged to evaluatewhether the measurement signal 1520.3 is within an acceptable range.When the measurement signal is within the acceptable range, the object620 can be properly removed from the electrostatic clamp 630. However,when the measurement signal is not within the acceptable range, thecontrol unit 1520 can take action.

The control unit 1520 may be arranged to send a control signal 1520.1 tothe power supply 1530. Power supply 1530 is arranged to provide power1530.2 to the ionizer device 1610. By providing power, the control unit1520 can switch the ionizer device 1610 on and may control the amount ofthe ionized flow of gas 1610.1. The control unit 1520 may control amovement of the ionizer device 1610 to move across the surface of theelectrostatic clamp 630 to provide the ionized flow of gas 1610 to theentire surface of the electrostatic clamp 630. Alternatively, theionizer device 1610 may be stationary and arranged to fill the spacesurrounding the electrostatic clamp 630 with ionized gas. In anotherexample, the ionizer device 1610 is stationary and the object table 620is moved by the control unit 1520 to provide the ionized flow of gas1610.1 to the entire surface of the electrostatic clamp 630. The ionizedgas neutralizes the residual charge on the electrostatic clamp 630.

Note that the ionized flow of gas 1610.1 should be able to make contactwith the electrostatic clamp 630. Therefore, preferably no object 620should be present on the electrostatic clamp 630 when operating theionizer device 1610. So when the control unit 1520 determines that theinformation signal is not within an acceptable range when unloading theobject 620, the object 620 is first unloaded before the ionizer device1610 is activated. The residual charge for the next object 620 will belower due to the neutralization of the residual charge by the ionizerdevice 1610. The acceptable range may be set such that when theinformation signal is just outside the acceptable range, the object 620can still be unloaded from the electrostatic clamp 630 withoutgenerating too much particles.

Alternatively, the control unit 1520 can send a control signal 1520.1 topower supply 1530 which provides power 1530.1 to the electrostatic clamp630. During clamping of the object 620, the control unit 1520 providesthe charge voltage, via the power supply 1530, to the electrostaticclamp 630 so as to clamp the object 620. When unloading, the controlunit 1520 stops providing the charge voltage. When, during unloading,the control unit 1520 evaluates that the information signal is not withan acceptable range, the control unit 1520 provides a discharge voltageto the electrostatic clamp 630. The discharge voltage may at leastpartly neutralize the residual charge. By neutralizing the residualcharge, unloading the object 620 may be done with an acceptableunloading force. Applying the discharge voltage by be especiallybeneficial when the unloading force is not sufficient to remove theobject 620 from the electrostatic clamp 630.

In some embodiments, the control unit 1520 may control both the ionizerdevice 1610 and the electrostatic clamp 630. In such an example, theobject table 1600 may be operated until the information signal exceedsthe acceptable range. The control unit 1520 controls the dischargevoltage to temporarily reduce the residual charge, so the object 620 canbe properly removed from the electrostatic clamp 620. After the object620 is removed from the electrostatic clamp 620, the control unit 1520stops providing the discharge voltage, which increases the residualcharge on the electrostatic clamp 620. Then, the ionizer device 1610 isused to neutralize the residual charge from the electrostatic clamp 620.These embodiments may have the advantage that the ionizer device 1610 isused as little as possible, so the operational use of the object table1600 is disturbed as little as possible.

The control unit 1520 may be arranged to receive an updated informationsignal representative of an updated residual force or an updatedresidual charge. The updated residual force or updated residual chargeis based on the residual force or residual charge and the dischargevoltage provided to the electrostatic clamp 630. When the control unit1520 provides the discharge voltage to the electrostatic clamp 630, themeasurement unit may detect a change in the measurement signal. Thechange in the measurement signal causes the measurement signal to formthe updated information signal. The control unit 1520 can use theupdated information signal to adjust the discharge voltage. The updatedinformation signal may be lower than the information signal. In thatcase, the polarity of the discharge voltage is correct. However, whenthe updated information signal is higher than the information signal,the polarity of the discharge voltage may be incorrect. As a result, thecontrol unit 1520 may apply a discharge voltage with an oppositepolarity or may apply a smaller discharge voltage.

The object table 1600 may comprise an unloading mechanism to unload theobject 620 from the electrostatic clamp 630. The unloading mechanism maycomprise a lift pin, like the pin-shaped member 650, or may comprise anyother type of mechanism such as a robot arm, a gripper to engage withthe bottom surface 620.1 of the object 620, or a vacuum gripper toengage with the top surface of the object 620. The measurement unit maybe arranged to monitor a lift force applied by the unloading mechanismon the object 620 to lift the object 620 from the electrostatic clamp630 during unloading. The lift force is based on the residual forceand/or the residual charge. To lift the object 620 from theelectrostatic clamp 630, the unloading mechanism needs to provide a liftforce to overcome the residual force and the gravity force due to theweight of the object 620. In general, the measurement unit as appliedmay comprise any type of sensor suitable to monitor the lift force. Forexample, the measurement unit may comprise a strain gauge attached tothe unloading mechanism, a power gauge that monitors the amount of powerneeded by the unloading mechanism to lift the object 620 from theelectrostatic clamp 630, and/or a force sensor that monitors the liftforce applied by the unloading mechanism.

In the example of FIG. 16, the unloading mechanism comprises thepin-shaped member 650. The pin-shaped member is provided with a forcesensor 1620, which forms part of the measurement unit. The force sensor1620 is arranged to monitor the lift force. When the pin-shaped member650 pushes up against the object 620 to unload the object 620 from theelectrostatic clamp 630, the lift force is created. The lift forcepropagates from the object 620 through the pin-shaped member 650 and theforce sensor 1620. The force sensor 1620 sends the force signal 1520.3to the control unit 1520.

The measurement unit may comprise a gap sensor 1630 arranged to providethe measurement signal based on the object 620 in a first state and theobject 620 in a second state. In the first state, the object 620 is heldby the electrostatic clamp 630. In the second state, the object is awayfrom the electrostatic clamp 630. For example, in the second state, theobject 610 is held by the unloading mechanism. When the object 620 is inthe first state, the gap sensor 1630 may measure a distance or gapbetween the gap sensor 1630 and the object 620, for example the bottomsurface 620.1. In the second state, the gap sensor 1630 may measure alarger distance or larger gap between the gap sensor 1630 and the object620, because the object 620 is further away from the electrostatic clamp630. In the second state, part of the object 620, for example a centrepart, may be further away from the electrostatic clamp 630, whereasanother part of the object 620, for example an edge part, is stillclamped to the electrostatic clamp 630.

The object 620 may perform a movement from the first state to the secondstate. The object 620 may jump from the first state to the second state.A change in the jump, may indicate a change of the residual force orresidual charge. The measurement signal may be representative themovement, for example a distance or a velocity of the movement. When theunloading mechanism starts providing the lift force to the object 620,the object 620 may remain in the first state. When the unloadingmechanism increases the lift force, the object 620 may suddenly detachfrom the electrostatic clamp 620 when the lift force exceeds theresidual force. The suddenly detachment may cause the object 620 move acertain distance or at a certain velocity. The higher the residualforce, the higher the certain distance or the certain velocity may be.The gap sensor 1630 may monitor the distance and/or velocity as ameasure of the residual force. Other known parameters that may influencethe distance or the velocity may be a stiffness, mass or damping of theunloading mechanism and a stiffness, mass or damping of the object 620.The gap sensor 1630 may comprise or may be a capacitive sensor. The gapsensor 1630 may be arranged in or at the electrostatic clamp 630.

As an alternative or in addition to the gap sensor 1630, a height sensor1640 may be used. The height sensor 1640 is arranged to determine aheight of the object 620, for example relative to the object table 1600or any other reference. The height sensor 1640 may be arranged todetermine a flatness of the object 620. The height sensor 1640 may bearranged as part of the object table 1600 or may be arranged separately,for example attached to a metrology frame. The height sensor 1640 may bean optical distance measurement sensor, such as an interferometer.

The control unit 1520 is arranged to compare the information signal witha threshold value. When the force signal does not exceed the thresholdvalue, the control unit 1520 does not need to take any action. When theinformation signal exceeds the threshold value, the control unit 1520 isarranged to take action, as described above.

The object table 1600 may be used in any suitable apparatus, such as oneof a particle beam apparatus, an electron beam apparatus, a scanningelectron microscope, an electron beam direct writer, an electron beamprojection lithography apparatus, an electron beam inspection apparatus,an electron beam defect verification apparatus, an electron beammetrology apparatus, a lithographic apparatus and a metrology apparatus.

The object 620 may be unloaded from the electrostatic clamp 630 byperforming the following method: unloading the object 620 from theelectrostatic clamp 630; and providing an ionized flow of gas to theelectrostatic clamp 630. Providing the ionized flow of gas may occurafter the object 620 has been unloaded from the electrostatic clamp 630.

The method may further comprise the steps of providing an informationsignal representative of a residual force or a residual charge. Theresidual force is applied by the electrostatic clamp 630 on the object620 during unloading of the object 620 from the electrostatic clamp 630.The residual charge is present on the electrostatic clamp 630 when nocharge voltage is applied to the electrostatic clamp 630. The methodfurther comprises the step of providing a discharge voltage to unclampthe object 620 from the electrostatic clamp 630 based on the informationsignal. The method may further comprise providing an ionized flow of gas1610.1 to the electrostatic clamp 630 based on the information signal.

Note that the power supply 1530 may be divided into separate powersupplies for each of the ionizer device 1610 and the electrostatic clamp630. The power supply 1530 may be integrated with the ionizer device1610 and/or the electrostatic clamp 630.

When clamping the object 620 onto the electrostatic clamp 630, thecharge voltage, also referred to as clamping voltage, needs to besufficient to properly clamp the object 620. Properly clamped means thatthe object 620 is pressed sufficiently flat against the electrostaticclamp 630. Before the object 620 is clamped, the object 620 may not beflat, but may be for example bowl-shaped, or saddle-shaped orumbrella-shaped. To flatten the object 620 against the electrostaticclamp 630, a high clamping voltage is required. However, if the clampingvoltage becomes higher, more particles are being generated. It istherefore desirable to apply a clamping voltage that is as low aspossible, while properly clamping the object 620 on the electrostaticclamp 630.

According to a fourth aspect of the present disclosure, this may be doneby clamping the object 620 on the electrostatic clamp 630 with thefollowing method: i) providing the object 620 on the electrostatic clamp630; ii) increasing a clamping voltage of the electrostatic clamp 630until a clamped state is detected in which the object 620 is clamped onthe electrostatic clamp 630; iii) determining a first clamping voltagebeing the clamping voltage at the clamped state; iv) providing a secondclamping voltage to the electrostatic clamp 630, which is less than thefirst clamping voltage. The clamped state may be detected when aclamping parameter is within a threshold. The clamping parameter maycomprise one of a flatness of the object, a change in shape of theobject caused by a change in the clamping voltage, and a center portionof the object being in contact with the electrostatic clamp.

With this method, the first clamping voltage is a clamping voltage thatis sufficient to properly clamp the object 620 on the electrostaticclamp 620. By doing so, one can avoid a voltage that is too high, i.e.an unnecessary high clamping voltage. Further, the inventors havediscovered that the first clamping voltage as applied is needed toflatten the object 620, but that a lower clamping voltage is sufficientto keep the object 620 flat on the electrostatic clamp 620. Therefore,after the first clamping voltage is applied, the second clamping voltagecan be set at a lower value than the first clamping voltage. As aresult, the second clamping voltage provides less particles than if thefirst clamping voltage would be kept.

FIG. 17 schematically presents the clamping voltage V as provided to theelectrostatic clamp 630 according to some embodiments of the presentdisclosure, as a function of time t. At t=0, the clamping voltage isincreased till at time t=t₁ the first clamping voltage V_(max) has beenreached. During the period 0−t₁, the deformation of the object 620 ismonitored, for example by the gap sensor 1630 or the height sensor 1640or any other suitable deformation sensor. As such, during the period0−t₁, the deformation of the object 620 is determined as a function ofthe clamping voltage increase. For a bow-shaped object 620, thedeformation may change much when the clamping voltage increase starts.But when the bow-shaped object 620 is flattened against theelectrostatic clamp 630, the deformation will no longer change or willchange only negligibly little in response to another clamping voltageincrease. So the deformation due to a clamping voltage increase can befound to be lower than a threshold, e.g. a predetermined threshold. Whena further clamping voltage increase does not lead to any substantialdifference in the deformation, it may be concluded that the object 620is flattened on the electrostatic clamp 630. With reference to FIG. 17,such condition occurs at t=t₁. The voltage as applied at t=t₁ may thenbe considered the first clamping voltage V_(max).

After providing the first clamping voltage V_(max) to the electrostaticclamp 630, the clamping voltage is decreased by providing a clampingvoltage decrease to the electrostatic clamp 630, and the deformation ofthe object 620 is determined caused by the clamping voltage decrease.This is repeated until the deformation is higher than a furtherthreshold. The clamping voltage is decreased until the clamped state isno longer detected. Then, a third clamping voltage V_(min) is determinedwhen the deformation is higher than the further threshold. The thirdclamping voltage V_(min) is the clamping voltage at which the clampedstate is no longer detected, for example the first value of the clampingvoltage at which the clamped state is no longer detected. The secondclamping voltage V_(final) is applied to the electrostatic clamp 630.The final clamping voltage V_(final) is higher than the third clampingvoltage V_(min).

As shown in FIG. 17, between t=t₁ and t=t₂, the clamping voltage isdecreased from the first clamping voltage V_(max) to the third voltageV_(min). At V_(min), the clamping force applied by the electrostaticclamp 630 has been reduced to the extent that the object 620 becomesunflat again. The determined deformation of the object 620 indicatesthat the object 620 has become unflat. Note that at t=t₂, the object 620may still be much more flat than at t=0.

In FIG. 17, between t=t₂ and t=t₃, the clamping voltage is increased tothe first clamping voltage V_(max). This may be done by simply applyingthe same clamping voltage as at time t₁ or by determining thedeformation of the object 620 until the deformation is again lower thanthe threshold. At time t₃, the object 620 is flattened against theelectrostatic clamp 630. After t₃, the clamping voltage is reduced tothe second clamping voltage V_(final) which is higher than the thirdclamping voltage V_(min) and lower than the first clamping voltageV_(max). This way, the object 620 maintains a proper flatness at a lowclamping voltage.

In the example described above, instead of determining the firstclamping voltage V_(max), the second clamping voltage V_(final) and thethird clamping voltage V_(min) based on a deformation of the object 620,one or more of those clamping voltages may be based on whether a centerportion of the object 620 is in contact with the electrostatic clamp630, or based on any other suitable clamping parameter.

The second clamping voltage V_(final) may be less than 150% of the thirdclamping voltage (V_(min)), for example less than 140% or less than 130%or less than 120% or less than 110% or less than 105%

FIG. 18 schematically presents the clamping voltage as provided to theelectrostatic clamp 630 according to another example of the presentdisclosure. Similarly to the example of FIG. 17, the clamping voltage isset to the first clamping voltage V_(max) at t=t₁, and to the thirdclamping voltage V_(min) at t=t₂. However, in these embodiments, theflatness at the third clamping voltage V_(min) is sufficient forprocessing the object 620. Therefore, between t=t₂ and t=t₃, theclamping voltage is increased from the third clamping voltage V_(min) tothe second clamping voltage V_(final). Because the flatness at the thirdclamping voltage V_(min) is sufficiently flat, it is not required toincrease the clamping voltage back to the first clamping voltage V_(max)to re-flatten the object 620. The clamping voltage is increased from thethird clamping voltage V_(min) to the second clamping voltage V_(final)to make sure that the object 620 remains clamped to the electrostaticclamp 630 during processing of the object 620.

FIG. 19 schematically presents the clamping voltage as provided to theelectrostatic clamp 630 according to yet another example of the presentdisclosure. When the first clamping voltage V_(max) is applied on timet₁, a clamping force between the object 620 and the electrostatic clamp630 is determined. The second clamping voltage V_(final) is providedbased on the clamping force. For example, a high clamping force at thefirst clamping voltage V_(max) may indicate a good clamping of theobject 620, so the second clamping voltage V_(final) may be setrelatively low. On the other hand, a low clamping force at the firstclamping voltage V_(max) may indicate a poor clamping of the object 620,so the second clamping voltage V_(final) may be set relatively high.

The object table 1600 may be arranged to set the clamping voltages asshown in FIGS. 17-19. The object table 1600 may comprise theelectrostatic clamp 630 for clamping the object 620 and may comprise thecontrol unit 1520 for providing the clamping voltage to theelectrostatic clamp 630. The object table 1600 may comprise themeasurement unit for providing the control unit 1520 with a measurementsignal representative of the deformation of the object 620. For example,the measurement unit comprises the gap sensor 1630. The gap sensor 1630may be arranged to determine a gap between the object and theelectrostatic clamp 630. Alternatively, the measurement unit is arrangedseparately from the object table 1600, such as the height sensor 1640which may be arranged on a metrology frame. The height sensor 1640 maybe arranged to determine the height of the object 620. The measurementunit may be arranged to determine whether the object 620 is flat orbow-shaped. Based on whether the object 620 is flat or bow-shaped, thecontrol unit 1520 may use the method of either FIG. 17, 18 or 19, or anyother suitable method.

FIG. 20 shows a further example of the present disclosure. An objecttable 2000, which may be the same as the object table 1600, except forthe following. The object table 2000 comprises a force sensor 2010. Theforce sensor 2010 may be the same as force sensor 1620. The force sensor2010 may extend beyond the surface of the electrostatic clamp 630 suchthat during time t₁, when the maximum clamping voltage V_(max) isapplied, the object 620 is partly clamped on the force sensor 2010. Thisway, the force sensor 2010 is arranged to detect the clamping force withwhich the object 620 is clamped on the electrostatic clamp 630 while thefirst clamping voltage V_(max) is applied to the electrostatic clamp630. After the clamping force at time t₁ is determined, the force sensor2010 may be moved in a down-position 2011, indicated by the dashed line.In the down-position 2011 the force sensor 2010 is retracted below thesurface of the electrostatic clamp 630, so the force sensor 2010 is notin contact with the object 620. When the force sensor 2010 is in thedown-position, the object 620 is able to be properly flattened on theelectrostatic clamp 630. Even though FIG. 20 shows a single force sensor2010, a plurality of force sensors 2010 may be applied distributed alongthe electrostatic clamp 630.

The control unit 1520 may comprise a machine learning unit arranged topredict the third clamping voltage V_(min) based on the clamping voltageand the clamping parameter. For example, the machine learning unit maytake into account, the change in clamping voltage over time, thedeformation over time, and/or the clamping force determined by the forcesensor 2010. The machine learning unit may be provided with informationof the object 620 that was acquired outside the object table 1600, suchas the material of the object 620, the dimensions of the object 620 andother measurement information about the object 620.

According to a fifth aspect of the present disclosure, there is provideda method of determining a residual charge of a clamping mechanism of anobject table. As indicated above, a residual charge may build up overtime on isolates surfaces such as surfaces of clamping mechanisms asapplied in lithographic apparatuses or inspection apparatuses such asparticle beam inspection tools or electron beam inspection tools. Such aresidual charge can e.g. build up over multiple object processing cyclesand may affect the clamping of an object to the clamping mechanism.Within the meaning of the present disclosure, a residual charge of aclamping mechanism may also refer to a charge on a surface or surfacesthat are close to the clamping surface of the clamping mechanism orsurfaces covering it. Such surfaces that surround or enclose or coverthe actual surface where the clamping force is generated, can beconsidered, within the meaning of the present disclosure, as being partof the clamping mechanism.

In some embodiments according to the fifth aspect of the presentdisclosure, it is proposed to determine or estimate the residual chargethat is present on a clamping mechanism by probing the clampingmechanism with a particle beam, e.g. a particle beam available in theinspection apparatus. In particular, in some embodiments, the presentdisclosure provides in a method of determining a residual charge of aclamping mechanism by impinging or probing the surface of the clampingmechanism using a particle beam, determining or detecting a response ofthe clamping mechanism to the impinging or probing of the surface, anddetermining the residual charge of the clamping mechanism based on thedetected response. FIG. 21 schematically shows a flowchart 2100 of themethod of determining the residual charge of a clamping mechanismaccording to the present disclosure. In a first step 2110, the methodcomprises impinging a surface of a clamping mechanism of an object tableusing a particle beam. Particles beams such as ion beams or electronbeams are generally known and applied in inspection apparatuses forinspecting surfaces or structures of objects such as semiconductorsubstrates. In such apparatuses, the object that is to be inspected istypically held, e.g. clamped, onto a clamping mechanism of an objecttable. During the inspection, a particle beam such as an electron beamis used to probe or impinge the object. Such probing or impinging mayresult in the generation of radiation and particles, emitted by theobject. As an example, the probing of an object by an electron beam maycause the object to emit secondary electrons or generate scatteredelectrons. In the method according to the present disclosure, theparticle beam, e.g. an electron beam, is used to impinge the surface ofthe clamping mechanism of an object table, rather than the object whichis typically held on the object table. Considering the typical layout ofan object table as applied in an inspection apparatus, the applicationof a particle beam to a surface of the clamping mechanism will requirethat the object table is not holding an object. In the absence of suchan object, the particle beam can be made to interact with the clampingmechanism rather than with an object.

In a second step 2120, the method according to the present disclosurecomprises detecting a response of the clamping mechanism to theimpinging of the surface by the particle beam. When a particle beam ismade to impinge on a surface of a clamping mechanism, e.g. anelectrostatic clamp, this may cause the generation of radiation and/orparticles that are emitted by the surface. As an example, an electronbeam impinging on the surface of a clamping mechanism may cause thegeneration of secondary electrons or scattered electrons. Said electronsmay, in some embodiments of the present disclosure, be detected by adetector. Such a detector may e.g. be configured to determine the amountof secondary electrons and/or the energy or energy spectrum of thesecondary electrons.

In a third step 2130, the method according to the present disclosurecomprises determining the residual charge of the clamping mechanism,based on the response. With respect to this third step, it can bepointed out that, when there is a residual charge on the clampingmechanism, the response to the particle beam, as detected by thedetector, will be different. This can be understood as follows: particlebeams such as electron beams are known to be used to inspect objectssuch a semiconductor substrates. During such inspection, a voltagedifference is applied between the object and the particle beam source,said voltage difference causing the particles to impinge on the objectwith a particular amount of energy. The response of the object to theimpinging particles, i.e. the effect of the impinging particles, willdepend on this amount of energy. The amount of energy of the particleswhen impinging the object may also be referred to as the landing energy(LE). Depending on the amount of energy the particles have whenimpinging the object, the response will be different; the amount ofradiation and/or emitted particles, such as secondary electrons, that isgenerated will depend on the landing energy. In case a charge, eitherpositive or negative, is present on the object that is examined, thiswill affect the landing energy of the particles that impinge on theobject. As such, a charge on the object will result in a differentresponse of the object to the impinging particle beam. In case aclamping mechanism, e.g. including an electrostatic clamp, is subjectedto a particle beam, one can thus determine the presence of a charge onthe clamping mechanism, based on the response of the clamping mechanismdue to the impinging particle beam. In some embodiments, the presence ofa charge on the clamping mechanism may be determined using e.g. anexperimental data set and/or a model representing a relationship betweenthe impinging particle beam and the response of the clamping mechanism.

As an alternative to determining the residual charge using a particlebeam, the use of an electrostatic voltmeter or voltage meter can also bementioned. In such an example, care should be taken to position thevoltmeter sufficiently close to the substrate or object table butsufficiently far from the high voltage area.

As will be appreciated by the skilled person, the presence of a chargeon the clamping mechanism may either cause a repelling or attractiveforce to the applied particle beam. In case the particle beam includesan electron beam, a positive charge on the clamping mechanism willresult in an attractive force on the electrons of the electron beam,causing an increased landing energy. In case of a negative charge on theclamping mechanism, the electrons of an electron beam will be repelled,causing a reduced landing energy.

In some embodiments of the present disclosure, the clamping mechanism asapplied comprises an electrostatic clamp. Such an electrostatic clampmay comprise one or more electrodes, e.g. embedded in a surface of theelectrostatic clamp.

In some embodiments, the step 2110 of impinging the surface of theclamping mechanism comprises impinging the surface at a plurality ofdifferent locations on the surface. By doing, the presence of a chargeon the clamping mechanism can be determined for the different locations.In such an example, the method according to the present disclosure caninclude determining a residual charge distribution across the surface ofthe clamping mechanism. In such an example, the method may e.g. comprisepositioning the clamping mechanism relative to the particle beams, e.g.using a stage apparatus, such that the particle beams impinge on thedifferent locations on the surface. Alternatively or in addition, theposition of the particle beam may be controlled as well, therebycontrolling at which the location on the clamping mechanism the particlebeam impinges.

Some embodiments according to a sixth aspect of the present disclosureprovide a particle beam apparatus that is configured to perform themethod 2100 according to the present disclosure. An example of such anapparatus is schematically shown in FIG. 22.

FIG. 22 schematically shows a particle beam apparatus 2200 according tosome embodiments of the present disclosure. In the example as shown, theparticle beam apparatus comprise a particle beam generator 2210, theparticle beam generator being configured to generate a particle beam2220, e.g. an ion beam or electron beam or multiple electron beams. Inthe example as shown, the apparatus 2200 further comprises an objecttable 2230 for holding an object 2240, the object table 2230 comprisinga clamping mechanism 2250 for clamping the object 2240 to the objecttable 2230. In some embodiments, the clamping mechanism 2250 can e.g.comprise an electrostatic clamp. Such an electrostatic clamp cancomprise one or more electrodes which can be connected to a powersource. In the example as shown, the apparatus 2200 further comprises adetector 2260. Such a detector 2260 may be configured to detect aresponse of the object 2240 when the object 2240 is subjected to theparticle beam 2220. In particular, the detector 2260 may be configuredto detect radiation and/or particles that are generated in response toan impinging of the object 2240 by the particle beam 2220. In responseto an impinging of the object by a particle beam, the object may e.g.emit secondary electrons or scattered electrons, which may be detectedby the detector 2260. In accordance with some embodiments of the presentdisclosure, the detector 2260 is configured to detect a response 2220.1of the clamping mechanism 2250, when the clamping mechanism is impingedby the particle beam 2220. In the example as shown, the apparatus 2200further comprises a control unit 2270. In accordance with someembodiments of the present disclsoure, the control unit is configuredto:

-   -   control the particle beam generator 2210 to cause a particle        beam 2220 to impinge on a surface 2250.1 of the clamping        mechanism 2250;    -   receive a detector signal 2260.1 from the detector 2260, the        detector signal 2260.1 representing the response of the clamping        mechanism 2250 to the impinging particle beam 2220, and    -   determine a residual charge on the clamping mechanism 2250,        based on the detector signal 2260.1.

The apparatus 2200 according to some embodiments of the presentdisclosure thus enables to determine a residual charge that is presenton a clamping mechanism. Having knowledge of such a residual charge mayfacilitate the unloading of an object, such as object 2240, when it hasbeen processed by the apparatus.

In some embodiments, the apparatus 2200 according to the presentdisclosure is configured to impinge the surface of the clampingmechanism at a plurality of different locations on the surface. By doingso, the presence of a charge on the clamping mechanism 2250 can bedetermined for the different locations. In order to do so, the apparatus2200 further comprises a stage or stage apparatus 2280 that isconfigured to position the object table 2230 relative to the particlebeam 2220. In particular, the stage apparatus 2280 can be configured toscan the clamping mechanism 2250 underneath the particle beam 2220. Sucha stage apparatus 2280 may e.g. comprise one or more motors or actuatorsfor positioning the object table 2230. In such an example, the controlunit 2270 may be configured to control the positioning of the objecttable 2230 as well, e.g. by generating suitable control signals for themotors or actuators of the stage or stage apparatus 2280. In someembodiments, the apparatus 2200 may further comprise a positionmeasurement system, e.g. an interferometer based or encoder basedmeasurement system, that is configured to measure a position of thestage apparatus 2280 relative to the particle beam generator 2210, orrelative to a reference or reference frame. When the residual charge ofthe clamping mechanism 2250 is determined at various locations, thecontrol unit 2270 can be configured to determine a residual chargedistribution across the surface of the clamping mechanism 2250.

The residual charge as determined may advantageously be used to controlor adjust the unloading process of an object 2240 that has beenprocessed on the object table 2230. As an example, the clampingmechanism as applied may e.g. be or comprise an electrostatic clamphaving one or more electrodes. The voltage as applied to the electrodesduring an unloading sequence of the object 2240 can be controlled takingaccount of the determined residual charge or residual chargedistribution. In some embodiments, a repelling force can be generatedbetween the object 2240 and the clamping mechanism 2250. This can e.g.be done by applying the same voltage to the object 2240 and the clampingmechanism 2250. In the example, the clamping mechanism 2250 may have oneor more electrodes to apply a voltage to the object 2240. This creates arepelling force between the object 2240 and the clamping mechanism 2250that is constant over distance. In some embodiments the force applied bya loading/unloading mechanism such as the loading/unloading mechanism550, 650 described above, may be removed first, and then the repellingforce may be applied between the object 2240 and the clamping mechanism2250. The repelling force may be smaller than the gravitational force.Then the loading/unloading mechanism may be used to lift the object2240. If the object 2240 still sticks to the clamping mechanism 2250, ahigher repelling force may be used. The higher repelling force may bestopped as soon as the object 2240 is moving. Therefore theloading/unloading mechanism may be set to touch the object 2240 andapply a small force. The position of the loading/unloading mechanism canbe measured. As soon as movement is detected (e.g. 50 μm displacement),the repelling force must be removed.

According to a seventh aspect of the present disclosure, there isprovided a method of reducing a surface charge of a clamping mechanism.Such a method is schematically described in the flowchart of FIG. 23. Inaccordance with the present disclosure, the method of reducing a surfacecharge of a clamping mechanism 2300 makes use of a particle beam forreducing a surface charge on a clamping mechanism, e.g. a clampingmechanism as applied on an object table of a particle beam apparatus. Inparticular, the method according to some embodiments of the presentdisclosure comprises a first step 2310 of generating a particle beam,the particle beam being configured to have a secondary emission yield(SEY) substantially equal to 1 in a surface of the clamping mechanism.As will be appreciated by the skilled person, when a particle beam isapplied to the surface of an object, e.g. an electron beam as applied toa semiconductor substrate for the purpose of inspection, an interactiontakes place between the electron beam and the object. Such aninteraction may e.g. cause the generation of secondary electrons. Theamount of generated secondary electrons will in general depend on theapplied voltage difference between the object and the particle beamsource. As such, by controlling the applied voltage difference, one cancontrol the amount of generated secondary electrons. In accordance withthe first step 2310 of the method 2300 according to the presentdisclosure, a particle beam is generated that has a secondary emissionyield (SEY) that is substantially equal to 1. Within the meaning of thepresent disclosure, the application of an electron beam having an SEYsubstantially equal to 1 implies that, on average, the number ofelectrons that is caused to interact with the object, corresponds to thenumber of electrons, referred to as a secondary electrons, that isemitted by the object. In order to arrive at such condition, i.e.whereby, on average, a secondary electron is emitted for each electronthat arrives at the object, the arriving or landing electron needs tohave a specific energy or energy level. The energy of an electron whenlanding on an object or surface of an object is in general referred toas the landing energy. As will be appreciated by the skilled person, thelanding energy (LE) of an electron depends a.o. on the applied voltagedifference between the object and the particle beam source. As such, thelanding energy of electrons of an electron beam can be controlled bycontrolling the voltage difference between the object and the particlebeam source. As such, one can control the applied voltage differencesuch that the landing energy (LE) of the electrons is such that an SEYsubstantially equal to 1 is obtained. In this respect, it can be pointedout that in an inspection apparatus for inspecting objects using aparticle beam, the object table holding the object may be equipped withone or more additional electrodes, e.g. electrodes that are differentfrom e.g. electrodes of a clamping mechanism Such electrodes may e.g. bereferred to as high-voltage plates. Such electrode or electrodes maye.g. be arranged along a circumference of the object or object table andmay be connected, during use, to a voltage source such as a high-voltagesource. When such electrodes are applied, it will be clear to theskilled person that these electrodes may also have an effect on thelanding energy (LE) of the electrons. Such electrodes should thus alsobe taken into account when determining the required voltage differenceto arrive at a suitable landing energy (LE), resulting in an SEYsubstantially equal to 1. As will be explained in more detail below, thelanding energy (LE) can also depend on a charge state of the object. Assuch, when referring to a voltage difference or landing energy LEresulting in an SEY of approx. 1, it is assumed that the object is in aneutral state, i.e. absent of surface charges. In some embodiments ofthe present disclosure, the first step 2310 of the method 2300 can bepreceded by or comprise the steps of:

-   -   determining a voltage difference between a source of the        particle beam and the clamping mechanism required to obtain the        SEY to be substantially equal to 1, and    -   generating the particle beam by applying the determined voltage        difference.

In a second step 2320 of the method 2300 according to the presentdisclosure, the surface of the clamping mechanism is impinged using theparticle beam as generated. by subjecting the surface of the clampingmechanism to a particle beam that is conditioned to have an SEYsubstantially equal to 1, it can be shown that a residual charge on theclamping mechanism can be reduced.

As will be appreciated by the skilled person, the LE which results in aSEY substantially equal to 1 will in general depend on the material thatis subjected to the particle beam. As such, in order to ensure that theparticle beam as generated results in an SEY substantially equal to 1,the material of the clamp mechanism and its SEY vs. LE characteristic,at least a required value of the LE to arrive at the SEY substantiallyequal to 1, should be known, in order to determine which voltagedifference to be applied between the particle beam source and theobject. It can be pointed out that such material characteristics, e.g. avalue of the LE corresponding to an SEY substantially equal to 1, an SEYvs. LE characteristic, can be determined empirically.

FIG. 24 schematically shows a typical SEY vs. LE characteristic 2400,i.e. a characteristic of the secondary electron yield (SEY) as afunction of the landing energy (LE) of electrons of a particle beam. Ascan be seen, the graph or characteristic 2400 has two energy levels, E1and E2 which result in an SEY substantially equal to 1. In case of acharacteristic have more than one energy level resulting in an SEYsubstantially equal to 1, it is important to select, as the requiredlanding energy LE, the energy level around which the graph orcharacteristic 2400 has a negative slope. As illustrated in FIG. 24, thegraph 2400 has a positive slope 2410 around energy level E1 and anegative slope 2420 around the energy level E2. Or, phrased differently,a derivative of the SEY with respect to the landing energy is positivearound energy level E1 and negative around energy level E2. When alanding energy E2 is applied, i.e. the energy value around which thegraph 2400 has a negative slope or negative derivative, this can resultin a reduction or neutralisation of any residual charges on a surface.This can be explained as follows: In case there are no surface chargespresent on the clamping mechanism, the application of a voltagedifference between the particle beam source and the clamping mechanismthat results in a landing energy E2 will cause, on average, the emissionof one secondary electron for each impinging electron. In case thisvoltage difference is applied to a surface having a positive charge, theelectrons will be attracted by said positive charge, resulting in anincreased landing energy LE, in particular a landing energy larger thanE2, e.g. landing energy E3. As can be seen from graph 2400, such alanding energy E3 will have a corresponding SEY smaller than 1. When theSEY is smaller than 1, the number of electrons supplied to the surfaceis larger than the number of electrons emitted by the surface, thusresulting in a reduction or neutralisation of the positive surfacecharge. In case the voltage difference is applied to a surface having anegative charge, the electrons will be repelled more or attracted lessby said negative charge, resulting in a reduced landing energy LE, inparticular a landing energy smaller than E2, e.g. landing energy E4. Ascan be seen from graph 2400, such a landing energy E4 will have acorresponding SEY larger than 1. When the SEY is larger than 1, thenumber of electrons supplied to the surface is smaller than the numberof electrons emitted by the surface, thus resulting in a reduction orneutralisation of the negative surface charge.

According to a eighth aspect of the present disclosure, the method 2300of reducing a surface charge of a clamping mechanism according to thepresent disclosure may advantageously be executed by a particle beamapparatus according to the present disclosure.

Such an apparatus configured to perform the method 2300 as schematicallyshown in FIG. 23, is shown in FIG. 25.

The apparatus 2500 as schematically shown may have a similar structureas the apparatus 2200 shown in FIG. 22. In the example as shown, theparticle beam apparatus 2500 comprises two particle beam generator 2510,2520. In the example as shown, the apparatus 2500 comprises a firstparticle beam generator 2510 which can e.g. be used to generate aparticle beam 2510.1 such as an electron beam for inspecting an object.The apparatus as shown further comprises a second particle generator2520 for generating a second particle beam 2520.1 which can e.g. be usedto neutralise a charge of an object prior to or after the object hasbeen inspected. Such a particle beam generator 2520 may also be referredto as a flood gun. Note that in general, both particle beam generators2510, 2520 may be oriented so as to generate a particle beam in adirection substantially perpendicular to the object 2540.

It is submitted that, in order to perform the method reducing a surfacecharge of a clamping mechanism, either one of the particle beamgenerators may be used.

In the example as shown, the apparatus 2500 further comprises an objecttable 2530 for holding an object 2540, the object table 2530 comprisinga clamping mechanism 2550 for clamping the object 2540 to the objecttable 2530. In the example as shown, the apparatus 2500 furthercomprises a control unit 2570, the control unit 2570 being configured toeither one of the particle beam generators 2510, 2520 to generate aparticle beam 2510.1, 2520.1 which is configured to have a secondaryemission yield (SEY) substantially equal to 1 in a surface of theclamping mechanism 2550. In some embodiments, the particle beam asgenerated by either one of the particle beam generators comprises one ormore electron beams. With reference to the above method 2300, this meansthat the control unit 2570 is configured to control the particle beamgenerator 2510 or 2520 such that a landing energy of the particles ofthe particle beam result in an SEY substantially equal to 1. Suchcontrol may involve, as explained above, controlling the voltagedifference between the source of the particle beam and the clampingmechanism to a suitable value, so as to obtain the desired landingenergy. In the example as shown, reference number 2570.1 e.g. indicatesa control signal generated by the control unit 2570 for controlling theparticle beam generator 2520 to generate the desired particle beam. Inaccordance with the present disclosure, the control unit 2570 is furtherconfigured to control the particle beam as generated to impinge thesurface of the clamping mechanism 2550. By doing so, as explained abovewith reference to FIG. 24, any residual charge on the clamping mechanism2550 can at least be reduced.

As a result of this reduction of the residual charge that may have beenbuild up, an unloading or unclamping of object that are inspected by theapparatus, can be facilitated. In the example as shown, the apparatus2500 further comprises a stage apparatus 2580 that is configured todisplace the object table 2530 relative to the particle beam generators.By doing so, the entire surface of the clamping mechanism 2550 orparticular portions thereof can be subject to the particle beam, inorder to reduce the residual charge on the clamping mechanism.

It can be submitted that the application of the method 2300 according tothe present disclosure, in order to reduce an residual charges on theclamping mechanism can be applied periodically. The method can e.g. beapplied at predetermined intervals, e.g. each time a particular numberof object is processed, e.g. inspected. Alternatively, or in addition,use can be made of indications or observations obtained during theprocessing of the objects. In particular, the above described methodsthat enable to determine the residual charge, or an indication thereofsuch as an increased unloading force, may be applied to trigger theapplication of the method 2300 to reduce the residual charge on aclamping mechanism such as clamping mechanism 2550. It can further besubmitted that the particle beam generator 2510 and/or 2520 can beprovided with focussing means for controlling a cross-section of theparticle beam impinging the surface of the clamping mechanism 2550. Thecross-section of the applied particle beam determines the density of theapplied particles to the clamping mechanism. The higher thecross-section of the particle beam, the lower the density of particles.Depending on the actual amount of residual charge that is present at aparticular location on the clamping mechanism, said location willrequire a certain amount of particles in order to neutralise the surfaceat said location. During a scanning process of the particle beam acrossthe clamping mechanism, the density of the applied particle beam shouldthus be taken into account when determining the scanning speed, in orderto ensure that the residual charge is sufficiently reduced. It has beendeduced by the inventors that, when a typical particle beam is appliedto a known clamping mechanism, e.g. a particle beam having across-section of 5-6 mm² and a clamping mechanism made from SiO₂, thetime to remove approx. 95% of a residual charge can be achieved within0.5 msec. Based on such parameters, a typical clamping mechanism can besubstantially neutralised in less than 20 sec.

It should be clear to the person skilled in the art that the method ofreducing the surface charge of the clamping mechanism according to theeighth aspect of present disclosure may be initiated based on thepredicted, estimated or measured residual force or residual chargeaccording the other aspects of the present disclosure as described inthis document.

According to a ninth aspect of the present disclosure, when an object2601 is held by an electrostatic clamp, the object 2601 may be chargedto a high voltage by one or more electrodes of the electrostatic clampthat protrude out of the object table 2602 and contact the lower surfaceof the object 2601. This may be a cause of an electric charge build-upthat holds the object 2601 to the object table 2602. In someembodiments, one or more further electrodes, that stick out of theobject table 2602 and contact the lower surface of the object 2601, areused to discharge any electric charge that has built up. The number offurther electrodes may be, for example, three. These embodiments isshown in FIG. 26. In FIG. 26, the electrode on the left has been used tocharge the object 2601 to a high voltage. The electrode on the right hasbeen brought into contact with the object 2601 in order to discharge theelectric charge build up. The electrode on the right may be connected atthe other end to the end that contacts the object 2601 to a groundpotential.

In some embodiments of the ninth aspect of the present disclosure, oneor more of the same electrodes that were used to charge the object 2601also discharge the object 2601. The discharge can therefore be improvedby increasing the number of electrodes used to charge the object 2601.The number of electrodes used to charge the object 2601 may be increasedin addition to using the one or more further electrodes to discharge theobject 2601.

According to a tenth aspect of the present disclosure, the object tableis cleaned so as to reduce the residual force that may continue to holdthe object 2601 to the object table 2602 due to the charge of theelectrostatic clamp after as the electrostatic clamp is controlled tostop holding the object 2601. For example, a cleaning stone may beprovided within the system to clean particles and/or contaminations onthe object table 2602, especially the surface of the electrostaticclamp, that may cause or enhance the charge build-up on the surface ofthe electrostatic clamp. The cleaning stone may be rotated and/ortranslated on the object table 2602, especially on the surface of theelectrostatic clamp. Alternatively, the cleaning stone may be stationaryand the object table 2602 moved so that it is rotated and/or translatedrelative to the cleaning stone. Both the object table 2602 and thecleaning stone may also be moved during a cleaning operation so that theobject table 2602, especially the surface of the electrostatic clamp, iscleaned. By providing the cleaning stone within the system, the cleaningoperations by the stone can be performed without the disruption causedby opening the system.

According to an eleventh aspect of the present disclosure, unloading ofthe object 2601 from the object table 2602 may be performed while theresidual force, or at least part of it, continue to hold the object 2601to the object table 2602 remains.

In some embodiments, the elevation pin positioning device 2605 is morepowerful than known implementations of an elevation pin positioningdevice. The elevation pin positioning device 2605 moves the elevationpins 2604 with an increased force so that a larger force is used toraise the object 2601. This is expected to achieve the effect of theforce applied by the elevation pins 2604 being sufficient to overcomethe residual force that is preventing the object 2601 from being raisedfrom the object table 2602.

In another example, the elevation pin positioning device 2605 vibratesthe elevation pins 2604 in the z direction so that the ends of theelevation pins 2604 impact the object 2601. The vibrational movement ofthe elevation pins 2604 that contact the object 2601 is expected toachieve the effect of the raising of the object 2601 above the objecttable 2602 by the elevation pins 2604 being easier.

In yet another example, the primary positioning device 506 vibrates in adirection in the x-y plane and/or in the x-y plane. The vibrationalmovement of the primary positioning device 506 applies a horizontalvibrational force to the object 2601 via the elevation pins 2604. Thisis expected to achieve the effect of the raising of the object 2601above the object table 2602 by the elevation pins 2604 being easier.

In yet another example, the table positioning device 2603 vibrates theobject table 2602 in one or more of a direction in the x-y plane, x-yplane and the z direction. Additionally, or alternatively, the vibratingmovement may also be rotational with the movement being about one ormore of the x-axis, the y-axis and the z-axis. The vibrational movementof the object table 2602 is expected to achieve the effect of theraising of the object 2601 above the object table 2602 by the elevationpins 2604 being easier.

In the above embodiments, the applied vibrational movement may be alongone or more of a direction in the x-y plane, the x-y plane and the zdirection. The applied vibrational movement may in addition, oralternatively, be rotational with the movement about any of the x-axis,the y-axis and the z-axis. The table positioning device 2603, elevationpin positioning device 2605 and a positioning device 2606 may make anypossible movements to apply the vibrations. For example, if thepositioning device 2606 is able to move the z direction then thepositioning device 2606 may apply a vibrational movement in the zdirection.

The vibrational movement may be dependent on any of a number ofdifferent types of control signal. For example, the signal may be asawtooth wave, sinusoidal wave, a noise signal and/or a pseudo-noisesignal.

Yet another example is shown in FIG. 28. In the yet another example, thetable positioning device 2603 moves the object table 2602 in the zdirection in a direction towards the elevation pin positioning device2605, as shown by the arrow in FIG. 28. The position of the elevationpins 2604 in the z direction may be mechanically locked so that themovement of the elevation pins 2604 in the z direction is prevented. Theeffect of this is that the object 2601 is forced against the ends of theelevation pins 2604 and this is expected to achieve the effect of theobject 2601 being removed, or made easier to remove, from the objecttable 2602.

In the example, the actuators of the table positioning device 2603 maybe more powerful than those of the elevation pin positioning device2605. The actuators of the table positioning device 2603 may also have alarger servo bandwidth than those of the elevation pin positioningdevice 2605 and this would allow the force applied by the tablepositioning device 2603 to be controlled faster than that of theelevation pin positioning device 2605. In an alternative implementationof the example, the table positioning device 2603 may move the objecttable 2602 in the z direction in a direction towards the elevation pinpositioning device 2605 and the elevation pin positioning device 2605may also control the elevation pins 2604 to move in the z directiontowards the object 2601.

In some embodiments, the table positioning device 2603 can be designedwith more powerful actuators than known table positioning devices andthe elevation pin positioning device 2605 to be designed with lesspowerful actuators than known elevation pin positioning devices. Thiswould simplify the design and implementation of the elevation pinpositioning device 2605. In addition to being able to apply a higherforce than the elevation pin positioning device 2605, the tablepositioning device 2603 may also comprise more actuators than theelevation pin positioning device 2605. A table positioning device 2603typically comprises 3 or 4 actuators for applying movements in the zdirection.

In yet another example, the object 2601 may effectively be peeled offthe object table 2602.

The example is shown in FIG. 29. In the example, the table positioningdevice 2603 rotates the object table 2602 about the x-axis and/ory-axis. The extent of the angle of rotation shown in FIG. 29 is large sothat the rotation is clearly illustrated. The actual angle of rotationmay be a lot less than that shown in FIG. 7. The position of one or moreof the elevation pins 2604 in the z direction may be mechanically lockedso the movement of the elevation pin(s) 2604 in the z direction isprevented. Alternatively, the position of one or more of the elevationpins 2604 in the z direction are not mechanically locked. One or more ofthe elevation pins 2604 may move in the z direction to increase theforce applied to the object 2601. The effect of this is that the object2601 is forced against the ends of one or more of the elevation pins2604. This is expected to achieve the effect of the object 2601 beingremoved, or made easier to remove, from the object table 2602 becausethe object 2601 may effectively be peeled off the object table 2602.

In some embodiments, one or more, but not all, of the elevation pins2604 are controlled to apply a force for raising the object 2601.Alternatively, all of the elevation pins 2604 may be controlled to applya force for raising the object 2601 but some of the elevation pins 2604controlled to apply a larger force than others. The effect of this isthat the elevation pins 2604 may apply a greater force on one side ofthe object 2601 than the other side of the object 2601. This is expectedto achieve the effect of the object 2601 being removed, or made easierto remove, from the object table 2602 because the object 2601 mayeffectively be peeled off the object table 2602.

In the above embodiments of the aspect of the present disclosure alsoinclude increasing the force that can be applied by each of theelevation pins 2604 over that applied according to known techniques.There may be any number of elevation pins 2604. For example, the numberof elevation pins 2604 may be 3, 6, 12, or more. Embodiments includemaking the ends of the elevation pins 2604 that contact the object 2601with an increased diameter from known elevation pins 2604. Both this,and increasing the number of elevation pins 2604, improves the spread ofthe force applied by the elevation pins 2604 over the surface of theobject 2601 and thereby reduces the risk of the object 2601 beingdamaged by the applied forces.

In the embodiments of the aspect of the present disclosure, thevibrations preferably have a small magnitude so that they result in onlysmall movements. Any of the techniques of the embodiments of the aspectof the present disclosure can be combined with each other. For examplethe elevation pins 2604 may be vibrated in the z direction as the tablepositioning device 2603 rotates the object table 2602 relative to the zdirection. In this implementation according to some embodiments thevibrated elevation pins 2604 would not be mechanically locked inposition.

Further embodiments may be described in the following clauses:

-   -   1. An object table comprising        -   a clamping mechanism for holding an object;        -   a loading/unloading mechanism configured to contact the            object to load or unload the object;        -   an electrical conductor configured to electrically connect            the object to a voltage source or an electrical ground to            apply a predetermined voltage to the object during at least            part of an unloading sequence of the object.    -   2. The object table according to clause 1, wherein the        electrical conductor is configured to form a low mechanical        stiffness connection when the object is held on the object        table.    -   3. The object table according to clause 1 or clause 2, wherein        the electrical conductor has a cross-section and wherein a        mechanical stiffness of the electrical conductor is lower than a        mechanical stiffness of an electric wire having the same        cross-section.    -   4. The object table according to clause 2, wherein the        mechanical stiffness is substantially zero at least part of a        time span when the object is held on the object table.    -   5. The object table according to any of the preceding clauses,        wherein the electrical conductor is configured to disconnect the        object from the voltage source or electrical ground when the        object is held on the object table.    -   6. The object table according to any of the preceding clauses,        wherein the electrical conductor comprises an electric wire        having a coil shaped portion.    -   7. The object table according to clause 6, wherein the coil        shaped portion comprises one or more windings or turns.    -   8. The object table according to clause 6 or 7, wherein the coil        shaped portion is arranged in a spiralling manner around a        pin-shaped member of the loading/unloading mechanism.    -   9. The object table according to clause 8, wherein an end of the        coil shaped portion is connected to the pin-shaped member.    -   10. The object table according to any of the preceding clauses,        further comprising an electrode.    -   11. The object table according to clause 10, wherein the        electrode is mounted at or near a top surface of the object        table.    -   12. The object table according to clause 10 or 11, wherein the        electrode substantially surrounds the clamping mechanism.    -   13. The object table according to any of the clauses 10 to 12,        wherein the electrical conductor is configured to electrically        connect the object to the electrode or to the electric ground.    -   14. The object table according to any of the clauses 10 to 13,        wherein an elevated voltage is configured to be applied to the        electrode during said at least part of an unloading sequence.    -   15. The object table according to any of the clauses 10 to 14,        wherein the electrode is electrically isolated from the object        during said at least part of an unloading sequence.    -   16. The object table according to any of the clauses 10 to 15,        wherein    -   17. the electrode is configured to be moved away from the object        before said at least part of an unloading sequence.    -   18. The object table according to any of the clauses 10 to 16,        wherein the clamping mechanism is configured to be moved away        from the electrode before said at least part of an unloading        sequence.    -   19. The object table according to any of the clauses 10 to 17,        wherein the loading/unloading mechanism comprises one or more        pin-shaped members for contacting the object to unload the        object.    -   20. The object table according to clause 18, wherein the        electrical conductor electrically connects at least one of the        one or more pin-shaped members with the electrode or the        electric ground.    -   21. The object table according to clause 19, wherein at least        part of the electric conductor is electrically shielded.    -   22. The object table according to clause 20, wherein the object        table comprises an electrical shield configured to shield at        least a part from the electrical conductor, the electrical        shield being electrically connected to the electrode.    -   23. The object table according to any of the clauses 1 to 8,        wherein the electrical conductor comprises a pin-shaped member        for contacting the object during the at least part of the        unloading sequence of the object.    -   24. The object table according to clause 22, further comprising        an electrode, wherein the electrical conductor is retractable        when the electrode is charged so as to avoid a discharge from        the electrode to the electrical conductor.    -   25. The object table according to clause 23, wherein the        electrical conductor is retractable to a position whereby the        electrical conductor is not surrounded by the clamp mechanism.    -   26. The object table according to any of the preceding clauses,        wherein the loading/unloading mechanism comprises one or more        pin-shaped members for contacting the object to unload the        object, at least one of the one or more pin-shaped members forms        at least a part of the electrical conductor.    -   27. The object table according to any of the clauses 22 to 25,        wherein the pin-shaped member comprises an ionisable gas.    -   28. The object table according to clause 26, wherein the        pin-shaped member is configured to receive an ionisable gas.    -   29. The object table according to clause 26 or 27, wherein the        ionisable gas comprises Ar or Ne.    -   30. The object table according to any of the preceding clauses,        wherein the electrical connector comprises a first connector        member and a second connector member, whereby the first        connector member and the second connector member are configured        to form an electrical connection during the at least part of the        unloading sequence of the object.    -   31. The object table according to clause 29, wherein the first        connector member or the second connector member comprises a        cantilever.    -   32. The object table according to clause 30, wherein the        cantilever comprises a bendable electrical conductor.    -   33. The object table according to clause 29, wherein the first        connector member comprises an aperture and the second connector        member is configured to protrude through the aperture.    -   34. The object table according to clause 32, wherein the first        connector member or the second connector member comprise a        plurality of brush like conductive wires to form the electrical        connection when the second connector member protrudes the        aperture.    -   35. The object table according to any of the preceding clauses,        wherein the electrical connector comprises the end effector        gripper.    -   36. An object table comprising        -   a support structure;        -   a clamping mechanism for holding an object, clamping            mechanism being arranged on the support structure;        -   a loading/unloading mechanism configured to contact the            object to load or unload the object;        -   an electrode mounted to the support structure;        -   an electrical conductor configured to electrically connect            the object to the electrode during at least part of an            unloading sequence of the object.    -   37. The object table according to clause 35, wherein the        electrode substantially surrounds the clamping mechanism.    -   38. The object table according to clause 35, wherein the        electrical conductor comprises a flexible portion, configured to        deform during the at least part of an unloading sequence of the        object, so as to maintain in contact with the object.    -   39. An object table comprising:    -   an electrostatic clamp configured to hold an object;    -   a measurement unit configured to determine one or more electric        characteristics of the electrostatic clamp, the one or more        electric characteristics being representative of one or more        charge states of the electrostatic clamp;    -   a control unit configured to control, before and/or during an        unloading of the object, one or more power supplies of the        electrostatic clamp, based on the determined one or more        electric characteristics.    -   40. The object table according to clause 38, wherein the        electrostatic clamp comprising one or more electrodes for        generating one or more electric fields between the object and        the electrostatic clamp.    -   41. The object table according to clause 38 or 39, wherein the        measurement unit is configured to measure one or more voltages        of the electrostatic clamp as the one or more electric        characteristic.    -   42. The object table according to clause 40 when dependent on        clause 39, wherein the one or more voltages of the electrostatic        clamp comprises one or more voltages of the one or more        electrodes.    -   43. The object table according to any of clauses 38 to 41,        wherein the measurement unit is configured to measure one or        more currents supplied to the electrostatic clamp as the one or        more electric characteristics.    -   44. The object table according to clause 42 when dependent on        clause 39, wherein the one or more currents supplied to the        electrostatic clamp comprises one or more currents supplied to        the one or more electrodes.    -   45. The object table according to any of the clauses 38 to 43,        wherein the measurement unit is configured to measure one or        more voltages of the object and/or one or more currents supplied        from/to the object.    -   46. The object table according to clause 44, wherein the object        table comprises a connecting pin configured to electrically        connect to the object when the object is positioned on the        object table and wherein the measurement unit is configured to        measure at least one of the one or more voltages of the object        via the connecting pin.    -   47. The object table according to any of the clauses 38 to 45,        wherein the object table comprises a loading/unloading mechanism        for loading and unloading the object.    -   48. The object table according to clause 46, wherein the        loading/unloading mechanism comprises a lifting mechanism        configured to lift the object from the electrostatic clamp.    -   49. The object table according to any of the clauses 38 to 47,        wherein the measurement unit is configured to measure the one or        more electric characteristic of the electrostatic clamp during a        loading or unloading of the object.    -   50. The object table according to any of the clauses 38 to 48,        wherein the object table further comprises an electrode        surrounding the electrostatic clamp.    -   51. The object table according to any of the clauses 38 to 49,        wherein the one or more charge states comprises a surface charge        on a surface of the electrostatic clamp or a surface charge        distribution on the surface of the electrostatic clamp.    -   52. The object table according to clause 50, wherein the control        unit is configured to control one or more power supplies of the        electrostatic clamp so as to generate one or more electric        fields that are configured to at least partly compensate an        electric field as generated by the surface charge to at least        partly compensate a sticking of the object caused by the surface        charge of the electrostatic clamp.    -   53. The object table according to any of clauses 1 to 37,        wherein the wherein the at least part of the electric conductor        is electrically shielded.    -   54. An object table for holding an object, comprising:    -   an electrostatic clamp for clamping the object on the object        table;    -   an ionizer device for providing an ionized flow of gas;    -   a control unit arranged to control the ionizer device to provide        the ionized flow of gas to the electrostatic clamp.    -   55. Object table of clause 53, wherein the control unit is        arranged to receive an information signal representative of a        residual force or a residual charge, wherein the residual force        is applied by the electrostatic clamp on the object during        unloading of the object from the electrostatic clamp, and        wherein the residual charge is an electrostatic charge present        on the electrostatic clamp when no charge voltage is applied to        the electrostatic clamp, and wherein the control unit is        arranged to control the ionizer device based on the information        signal.    -   56. Object table according to clause 54, comprising a        measurement unit for providing a measurement signal        representative of the residual force or the residual charge,        wherein the information signal comprises the measurement signal,        and wherein the control unit is arranged to control the ionizer        device based on the measurement signal.    -   57. An object table for holding an object, comprising:    -   an electrostatic clamp for clamping the object on the object        table;    -   a control unit arranged to provide to the electrostatic clamp a        charge voltage to clamp the object on the electrostatic clamp        and a discharge voltage to unclamp the object from the        electrostatic clamp,    -   wherein the control unit is arranged to receive an information        signal representative of a residual force or a residual charge,    -   wherein the control unit is arranged to provide the discharge        voltage based on the information signal.    -   58. Object table according to clause 56, wherein the discharge        voltage has a polarity opposite to the charge voltage.    -   59. Object table according to clauses 56 or 57, comprising an        ionizer device for providing an ionized flow of gas to the        electrostatic clamp, wherein the control unit is arranged to        control the ionizer device based on the information signal.    -   60. Object table according to clauses 53-58, wherein the        information signal comprises at least one of measurement        information and/or estimated information.    -   61. Object table according to any of clauses 56-59, wherein,        during unloading, the control unit is arranged to receive an        updated information signal representative of an updated residual        force or an updated residual charge, wherein the control unit is        arranged to adjust the discharge voltage based on the updated        information signal or the control unit is arranged to adjust the        control of the ionizer device based on the updated information        signal.    -   62. Object table according to any clauses 53-60, comprising an        unloading mechanism, wherein the measurement unit is arranged to        monitor a lift force applied by the unloading mechanism on the        object to lift the object from the electrostatic clamp during        unloading, wherein the measurement signal comprises a        measurement of the lift force.    -   63. Object table according to clause 61, wherein the measurement        unit comprises a force sensor to provide the measurement of the        lift force.    -   64. Object table according to clause 62, wherein the unloading        mechanism comprises at least one lift pin arranged to push the        object away from the electrostatic clamp during unloading,        wherein the lift pin comprises the force sensor.    -   65. Object table according to any of clauses 53-63, wherein the        measurement unit comprises a gap sensor arranged to provide the        measurement signal based on the object in a first state and the        object in a second state, wherein, in the first state, the        object is on the electrostatic clamp, and wherein, in the second        state, the object is away from the electrostatic clamp.    -   66. Object table according to clause 64, wherein the object        performs a movement from the first state to the second state,        wherein the measurement signal is representative of the        movement.    -   67. Object table according to clause 64 or 65 wherein the gap        sensor comprises a capacitance sensor.    -   68. Object table according to any of clauses 53-66, wherein the        control unit is arranged to compare the information signal with        a threshold value.    -   69. An apparatus comprising the object table of any of clauses        53-67, wherein the apparatus is one of a particle beam        apparatus, an electron beam apparatus, a scanning electron        microscope, an electron beam direct writer, an electron beam        projection lithography apparatus, an electron beam inspection        apparatus, an electron beam defect verification apparatus, an        electron beam metrology apparatus, a lithographic apparatus and        a metrology apparatus.    -   70. Method for unloading an object from an electrostatic clamp,        the method comprising:

unloading the object from the electrostatic clamp; providing an ionizedflow of gas to the electrostatic clamp.

-   -   71. Method for unloading an object from an electrostatic clamp,        the method comprising:    -   providing an information signal representative of a residual        force or a residual charge,    -   wherein the residual force is applied by the electrostatic clamp        on the object during unloading of the object from the        electrostatic clamp, and wherein the residual charge is present        on the electrostatic clamp when no charge voltage is applied to        the electrostatic clamp;    -   providing a discharge voltage to unclamp the object from the        electrostatic clamp based on the information signal.    -   72. Method according to clause 70, comprising the step of        providing an ionized flow of gas to the electrostatic clamp        based on the information signal.    -   73. Method for clamping an object on an electrostatic clamp, the        method comprising:    -   i) providing the object on the electrostatic clamp;    -   ii) increasing a clamping voltage until a clamped state is        detected in which the object is clamped on the electrostatic        clamp;    -   iii) determining a first clamping voltage (V_(max)) being the        clamping voltage at the clamped state;    -   iv) providing a second clamping voltage (V_(final)) to the        electrostatic clamp, which is less than the first clamping        voltage (V_(max)).    -   74. Method of clause 72, wherein the clamped state is detected        when a clamping parameter is within a threshold.    -   75. Method of clause 73, wherein, the clamping parameter        comprises one of a flatness of the object, a change in shape of        the object caused by a change in the clamping voltage, and a        center portion of the object being in contact with the        electrostatic clamp.    -   76. Method of clause 74, comprising in between steps iii) and        iv):    -   v) decreasing the clamping voltage until the clamped state is no        longer detected;    -   vi) determining a third clamping voltage (V_(min)) being the        clamping voltage at which the clamped state is no longer        detected;    -   vii) providing the second clamping voltage (V_(final)) to the        electrostatic clamp, which is higher than the third clamping        voltage (V_(min)).    -   77. Method of clause 75, comprising in between steps vi) and        vii):    -   viii) increasing the clamping voltage to the first clamping        voltage (V_(max)).    -   78. Method of clause 75 or 76, wherein the second clamping        voltage (V_(final)) is less than 150% of the third clamping        voltage (V_(min)), for example less than 140% or less than 130%        or less than 120% or less than 110% or less than 105%.    -   79. Method of clause 72, comprising:    -   ix) during step iii), determining a clamping force between the        object and the electrostatic clamp, and    -   x) providing the second clamping voltage (V_(final)) based on        the clamping force.    -   80. An object table for holding an object, wherein the object        table is arranged to perform the method of the preceding clauses        72 to 78.    -   81. Object table of clause 79, wherein the object table        comprises:    -   the electrostatic clamp for clamping the object; and    -   a control unit for providing the clamping voltage to the        electrostatic clamp.    -   82. Object table according to clause 80, comprising a        measurement unit for providing to the control unit a measurement        signal representative of the clamping parameter.    -   83. Object table according to clause 81, wherein the measurement        unit is arranged to determine whether the object is flat or        bow-shaped.    -   84. Object table according to clause 81 or 82, wherein the        measurement unit is arranged to determine a gap or a capacitance        between the object and the electrostatic clamp.    -   85. Object table according to any of clauses 81-83, wherein the        measurement unit is arranged to determine a height of the        object.    -   86. Object table according to any of clauses 79-84, comprising a        force sensor, wherein in during step iii) the force sensor is        arranged to detect a clamping force with which the object is        clamped on the electrostatic clamp while the first clamping        voltage (V_(max)) is applied to the electrostatic clamp.    -   87. Object table according to clause 85, wherein the force        sensor is movable so as to be in contact with the object during        step iii) and so as to be not in contact with the object during        step iv).    -   88. Object table according to clauses 79-86, wherein the control        unit comprises a machine learning unit arranged to predict the        third clamping voltage (V_(min)) based on the clamping voltage        and the clamping parameter.    -   89. An apparatus comprising the object table of any of clauses        79-87, wherein the apparatus is one of a particle beam        apparatus, an electron beam apparatus, a scanning electron        microscope, an electron beam direct writer, an electron beam        projection lithography apparatus, an electron beam inspection        apparatus, an electron beam defect verification apparatus, an        electron beam metrology apparatus, a lithographic apparatus and        a metrology apparatus.    -   90. The apparatus of clause 88, comprising a lifting pin for        moving the object from and onto the electrostatic clamp, wherein        the control unit is arranged to determine the clamping parameter        based on a position of the lifting pin.    -   91. The apparatus of clause 88 or 89, comprising a height sensor        for determining a height of the object, wherein the control unit        is arranged to determine the clamping parameter based on a        height signal from the height sensor.    -   92. A method of determining a residual charge of a clamping        mechanism of an object table, the method comprising:        -   impinging a surface of the clamping mechanism using a            particle beam;        -   detecting a response of the clamping mechanism caused by the            impinging of the surface, and        -   determining the residual charge of the clamping mechanism,            based on the response.    -   93. The method according to clause 91, wherein the particle beam        comprises one or more electron beams.    -   94. The method according to clause 91 or 92, wherein the step of        detecting the response comprises detecting secondary electrons        or scattered electrons emitted by the clamping mechanism.    -   95. The method according to clause 93, wherein the step of        detecting secondary electrons comprises measuring an energy        spectrum of the secondary electrons.    -   96. The method according any of the clauses 91 to 94, wherein        the step of impinging the surface of the clamping mechanism is        preceded by the step of positioning the clamping mechanism in an        operating range of the particle beam.    -   97. The method according to any of the clauses 91 to 95, wherein        the clamping mechanism comprises an electrostatic clamp.    -   98. The method according to clause 96, wherein the electrostatic        clamp comprises one or more electrodes.    -   99. The method according to clause 97, wherein the one or more        electrodes are embedded in the surface of the electrostatic        clamp.    -   100. The method according to any of the clauses 91 to 98,        wherein the step of impinging the surface comprises impinging        the surface at a plurality of locations on the surface, and        wherein the step of determining the residual charge comprises        determining a residual charge distribution across the surface of        the clamping mechanism.    -   101. The method according to any of the clauses 91 to 99,        further comprising the step of applying one or more voltage to        the clamping mechanism to at least partially cancel the residual        charge of the clamping mechanism, based on the response.    -   102. A particle beam apparatus configured to perform the method        according to any of the clauses 91 to 100.    -   103. A particle beam apparatus comprising:        -   a particle beam generator;        -   an object table for holding an object, the object table            comprising a clamping mechanism for clamping the object to            the object table;        -   a detector;        -   a control unit, the control unit being configured to:            -   control the particle beam generator to cause a particle                beam to impinge on a surface of the clamping mechanism;        -   the detector being configured to detect a response of the            clamping mechanism, caused by the clamping mechanism being            impinged by the particle beam;        -   the control unit further being configured to:            -   receive a detector signal from the detector, the                detector signal representing the response of the                clamping mechanism;            -   determine a residual charge on the clamping mechanism,                based on the detector signal.    -   104. The particle beam apparatus according to clause 102,        wherein the particle beam comprises one or more electron beams.    -   105. The particle beam apparatus according to clause 102 or 103,        wherein the apparatus further comprises a positioning device for        positioning the object table relative to the particle beam        generator.    -   106. The particle beam apparatus according to any of the clauses        102 to 104, wherein the response comprises secondary electrons        or scattered electrons emitted by the clamping mechanism.    -   107. The particle beam apparatus according to clause 105,        wherein the detector is configured to measure an energy spectrum        of the secondary electrons.    -   108. The particle beam apparatus according any of the clauses        102 to 106, wherein the positioning device is configured to        position the clamping mechanism in an operating range of the        particle beam, prior to the impinging of the surface by the        particle beam.    -   109. The particle beam apparatus according to any of the clauses        102 to 107, wherein the clamping mechanism comprises an        electrostatic clamp.    -   110. The method according to clause 108, wherein the        electrostatic clamp comprises one or more electrodes.    -   111. The method according to clause 109, wherein the one or more        electrodes are embedded in the surface of the electrostatic        clamp.    -   112. The method according to any of the clauses 102 to 110,        wherein the control unit is configured to control the particle        beam to impinge the surface at a plurality of locations on the        surface, and wherein the control unit is configured to determine        a residual charge distribution across the surface of the        clamping mechanism.    -   113. The method according to any of the clauses 102 to 111,        wherein the control unit is further configured to further        comprising the step of applying one or more voltage to the        clamping mechanism to at least partially cancel the residual        charge of the clamping mechanism, based on the response.    -   114. A method of reducing a surface charge of a clamping        mechanism, the method comprising:        -   generating a particle beam, the particle beam being            configured to have a secondary emission yield (SEY)            substantially equal to 1 in a surface of the clamping            mechanism;        -   impinging the surface of the clamping mechanism using the            particle beam.    -   115. The method according to clause 113, wherein the SEY is        considered to be substantially equal to 1 when the surface of        the clamping mechanism is in a neutral state.    -   116. The method according to clause 113 or 114, whereby the step        of generating the particle beam comprises:        -   determining a voltage difference between a source of the            particle beam and the clamping mechanism required to obtain            the SEY to be substantially equal to 1, and        -   generating the particle beam by applying the determined            voltage difference.    -   117. The method according to clause 115, whereby the voltage        difference is based on a material characteristic of the clamping        mechanism.    -   118. The method according to clause 115 or 116, whereby the        voltage difference is selected to have particles of the particle        beam having a landing energy (LE) corresponding to the SEY to be        substantially equal to 1 when the surface of the clamping        mechanism is in a neutral state.    -   119. The method according to clause 100, whereby the voltage        difference is selected based on a derivative of the SEY with        respect to the LE of the clamping mechanism, whereby the        derivative of the SEY is negative around the landing energy        (LE).    -   120. The method according to any of the clauses 113 to 118,        further comprising scanning the particle beam across the surface        of the clamping mechanism.    -   121. The method according to clause 119, whereby the particle        beam comprises one or more electron beams.    -   122. The method according to any of the clauses 113 to 120,        wherein a material of the clamping mechanism comprises SiO₂.    -   123. A particle beam apparatus comprising:        -   a particle beam generator;        -   an object table for holding an object, the object table            comprising a clamping mechanism for clamping the object to            the object table;        -   a control unit, the control unit being configured to:            -   control the particle beam generator to generate a                particle beam, the particle beam being configured to                have a secondary emission yield (SEY) substantially                equal to 1 in a surface of the clamping mechanism;            -   control the particle beam to impinge the surface of the                clamping mechanism.    -   124. The particle beam apparatus according to clause 122,        wherein the control unit is further configured to:    -   determining a voltage difference between a source of the        particle beam and the clamping mechanism, the voltage difference        being selected to cause the SEY to be substantially equal to 1,        and    -   control the particle beam generator to generate the particle        beam by applying the determined voltage difference.    -   125. The particle beam apparatus according to clause 122 or 123,        wherein the particle beam generator comprises an electron beam        generator.    -   126. The particle beam apparatus according to any of the clauses        122 to 124, wherein the particle beam generator comprises a        flood gun.    -   127. The particle beam apparatus according to any of the clauses        122 to 125, further comprising a stage apparatus configured to        position the object table relative to the particle beam.    -   128. The particle beam apparatus according to any of the clauses        122 to 126, wherein the SEY is considered to be substantially        equal to 1 when the surface of the clamping mechanism is in a        neutral state.    -   129. The particle beam apparatus according to clause 123 to 127        referring to clause 123, whereby the voltage difference is based        on a material characteristic of the clamping mechanism.    -   130. The particle beam apparatus according to clause 127 or 128,        whereby the voltage difference is selected to have particles of        the particle beam having a landing energy (LE) corresponding to        the SEY to be substantially equal to 1 when the surface of the        clamping mechanism is in a neutral state.    -   131. The particle beam apparatus according to clause 129,        whereby the voltage difference is selected based on a derivative        of the SEY with respect to the LE of the clamping mechanism,        whereby the derivative of the SEY is negative around the landing        energy (LE).    -   132. The particle beam apparatus according to any of the clauses        122 to 130, whereby the control unit is configured to control        the particle beam generator to scan the particle beam across the        surface of the clamping mechanism.    -   133. The particle beam apparatus according to clause 131,        whereby the particle beam comprises one or more electron beams.    -   134. The particle beam apparatus according to any of the clauses        122 to 132, wherein a material of the clamping mechanism        comprises SiO₂.    -   135. An electron beam apparatus comprising a particle beam        apparatus according to any of the clauses 122 to 132.    -   136. An electron beam apparatus comprising a particle beam        apparatus according to any of the clauses 122 to 132, the        particle beam apparatus comprising a first electron beam        generator for processing the object and a second electron beam        generator to at least partially cancel or neutralize a charge of        the surface of the clamping mechanism.    -   137. An object table for holding an object comprising:    -   an electrostatic clamp arranged to clamp the object on the        object table;    -   one or more elevation pins arranged to lift the object up from        the object table; and    -   a controller configured to send an actuation signal to one or        more elevation pin positioning device so as to vibrate the one        or more elevation pins and/or at least part of the object table.    -   138. The object table according to clause 136, wherein vibrating        the one or more elevation pins and/or the at least part of the        object table comprises:    -   moving the one or more elevation pins and/or the at least part        of the object table in a direction that is substantially        orthogonal to a surface of the electrostatic clamp; and/or    -   moving the one or more elevation pins and/or the at least part        of the object table in a direction that is substantially        parallel to the surface of the electrostatic clamp.    -   139. The object table according to clause 136 or 137, further        comprising a positioning system that comprises the one or more        actuators.    -   140. The object table according to clause 138, wherein the        positioning system comprises:    -   an elevation pin positioning device comprising at least one        actuator arranged to move each of the one or more elevation        pins;    -   an object table positioning device comprising at least one        actuator arranged to move the electrostatic clamp; and/or    -   a primary positioning device comprising at least one actuator        arranged to move the table positioning device.    -   141. An object table comprising:    -   an electrostatic clamp arranged to clamp the object on the        object table;    -   one or more elevation pins arranged to lift the object up from        the object table; and    -   a controller configured to send an actuation signal to one or        more actuators to move at least part of the object table so        that, when the object is on the electrostatic clamp, the object        is forced against the one or more elevation pins to thereby move        the object away from the electrostatic clamp.    -   142. The object table according to clause 140, further        comprising a locking system arranged to prevent the one or more        elevation pins from moving when the object is forced against the        one or more elevation pins.    -   143. The object table according to clause 141, wherein the        locking system comprises one or more mechanical locks.    -   144. The object table according to any of clauses 140 to 142,        wherein one or more elevation pins and/or at least part of the        object table are arranged to vibrate in response to an actuation        signal from the controller.    -   145. An object table comprising:    -   an electrostatic clamp arranged to clamp the object on the        object table;    -   a plurality of elevation pins arranged to lift the object up        from the object table; and    -   a controller configured to send an actuation signal to one or        more actuators to control the movement of at least part of the        object table and/or at least one of the plurality of elevation        pins such that, when the object is on the surface of the support        structure, at least one of the elevation pins contact the object        and not all of the plurality of elevation pins simultaneously        apply the same force against the object.    -   146. The object table according to clause 144, wherein, when the        object is on the electrostatic clamp, the object table and/or        plurality of elevation pins are arranged to tilt the object        relative to the electrostatic clamp when removing the object        from the electrostatic clamp in response to the actuation signal        sent by the controller.    -   147. The object table according to clause 145, further        comprising a locking system arranged to prevent the one or more        elevation pins from moving when the object is rotated.    -   148. The object table according to clause 146, wherein the        locking system comprises one or more mechanical locks.    -   149. The object table according to any of clauses 144 to 147,        wherein the controller is configured to send an actuation signal        to one or more actuators to control the movement of the object        table and/or plurality of pins such that, when the object is on        the electrostatic clamp, the object is rotationally vibrated.    -   150. The object table according to any of clauses 144 to 148,        wherein one or more elevation pins and/or at least part of the        object table are arranged to vibrate in response to the        actuation signal and/or a further actuation signal from the        controller.    -   151. An object table comprising:    -   an electrostatic clamp arranged to clamp the object on the        object table; and    -   one or more electrodes arranged to charge the object;    -   wherein a first set of the one or more electrodes is arranged to        apply an electric charge to the object; and    -   a second set of the one or more electrodes is arranged to        electrically discharge the object.    -   152. The object table according to clause 150, wherein the        second set of the one or more electrodes is used to electrically        discharge the object but not used to apply an electric charge to        the object.    -   153. The object table according to clause 150 or 151, further        comprising one or more elevation pins configured to lift the        object away from the electrostatic clamp.    -   154. An object table comprising:    -   an electrostatic clamp arranged to clamp the object on the        object table; and    -   a cleaning device;    -   wherein the cleaning device is arranged to clean the        electrostatic clamp.    -   155. The object table according to clause 153, wherein, when in        use with an object, the cleaning device is arranged to clean a        surface of the object.    -   156. The object table according to clause 153 or 154, further        comprising one or more elevation pins configured to lift the        object away from the electrostatic clamp.    -   157. An object table for holding an object, comprising:    -   an electrostatic clamp arranged to clamp the object on the        object table;    -   a neutralizer arranged to neutralize a residual charge of the        electrostatic clamp;    -   a control unit arranged to control the neutralizer.    -   158. The object table according to clause 156, wherein the        control unit is arranged to receive an information signal        representative of a residual force or the residual charge,        wherein the residual force is applied by the electrostatic clamp        on the object during unloading of the object from the        electrostatic clamp, wherein the residual charge is an        electrostatic charge present on the electrostatic clamp when no        voltage is applied to the electrostatic clamp and wherein the        control unit is arranged to control the neutralizer based on the        information signal.    -   159. The object table according to clause 157, wherein the        information signal comprises at least one of measurement        information, estimated information and internal signal        information.    -   160. The object table according to clause 157 or 158, further        comprising a measurement unit, wherein the measurement unit        comprises a force sensor configured to provide a measurement of        the residual force and/or a gap sensor configured to provide a        measurement of a gap between the object and the electrostatic        clamp.    -   161. The object table according to clause 157 or 158, further        comprising a further measurement unit, wherein the further        measurement unit is configured to determine the information        signal representing the residual charge of the electrostatic        clamp, the one or more electric characteristics being        representative of the residual charge of the electrostatic        clamp, and wherein the further measurement unit is configured to        measure one or more voltages of the electrostatic clamp as the        one or more electric characteristics, or the further measurement        unit is configured to measure one or more currents supplied to        the electrostatic clamp as the one or more electric        characteristics.    -   162. The object table according to clause 157 or 158, further        comprising    -   a particle beam generator configured to generate a particle        beam;    -   a detector configured to detect the particle beam;    -   wherein the control unit is configured to control the particle        beam generator to impinge the particle beam on a surface of the        electrostatic clamp,    -   wherein the detector is configured to detect a response of the        electrostatic clamp, caused by the electrostatic clamp being        impinged by the particle beam, and    -   wherein the control unit is further configured to: receive a        detector signal from the detector, the detector signal        representing the response of the electrostatic clamp, and    -   determine the information signal representing the residual        charge on the electrostatic clamp based on the detector signal.    -   163. The object table according to clause 156, wherein the        neutralizer comprises a power source configured to apply a        discharge voltage to the electrostatic clamp, and wherein the        control unit is arranged to control the discharge voltage to the        power source.    -   164. The object table according to any of clauses 157 to 161,        wherein the neutralizer comprises a power source configured to        apply a discharge voltage to the electrostatic clamp, and        wherein the control unit is arranged to control the discharge        voltage to the power source based on the information signal        representing the residual charge.    -   165. The object table according to any of clauses 157 to 161,        wherein the neutralizer is an ionizer device arranged to provide        an ionized flow of gas and wherein the control unit is arranged        to control the ionizer device to provide the ionized flow of gas        to the electrostatic clamp.    -   166. An apparatus comprising the object table according to any        of clauses 156 to 164, wherein the apparatus is one of a        particle beam apparatus, an electron beam apparatus, a scanning        electron microscope, an electron beam direct writer, an electron        beam projection lithography apparatus, an electron beam        inspection apparatus, an electron beam defect verification        apparatus, an electron beam metrology apparatus, a lithographic        apparatus and a metrology apparatus.    -   167. Method for unloading an object from an electrostatic clamp,        the method comprising:    -   unloading the object from the electrostatic clamp;    -   neutralizing a residual charge of the electrostatic clamp        before, during, and/or after the unloading step.    -   168. Method according to clause 166, the method comprising:    -   providing an information signal representative of a residual        force or the residual charge, wherein the residual force is        applied by the electrostatic clamp on the object during        unloading of the object from the electrostatic clamp, wherein        the residual charge is present on the electrostatic clamp when        no charge voltage is applied to the electrostatic clamp, and        wherein the step neutralizing the residual charge is based on        the information signal.    -   169. Method according to clause 167, wherein the information        signal comprises at least one of measurement information,        estimated information and internal signal information.    -   170. Method according to any of clauses 166 to 168 the method        comprising a step of providing an ionized flow of gas to the        electrostatic clamp based on the information signal and/or a        step of providing a discharge voltage to the electrostatic clamp        based on the information signal.

Although specific reference is made to the electrostatic clamp in thisdocument, the present disclosure may be applicable to any electricalclamps which utilize similar electrical phenomenon.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications. Possible other applications include the manufactureof integrated optical systems, guidance and detection patterns formagnetic domain memories, flat-panel displays, liquid-crystal displays(LCDs), thin-film magnetic heads, etc.

Although specific reference may be made in this text to embodiments ofthe present disclosure in the context of a lithographic apparatus,embodiments of the present disclosure may be used in other apparatus.Embodiments of the present disclosure may form part of a mask inspectionapparatus, a metrology apparatus, or any apparatus that measures orprocesses an object such as a wafer (or other substrate) or mask (orother patterning device). These apparatus may be generally referred toas lithographic tools. Such a lithographic tool may use vacuumconditions or ambient (non-vacuum) conditions.

Although specific reference may be made in this text to embodiments ofthe present disclosure in the context of an inspection apparatus, theobject table may be suitable for use in: an electron beam apparatus, ascanning electron microscope, an electron beam direct writer, anelectron beam projection lithography apparatus, an electron beaminspection apparatus, an electron beam defect verification apparatus, oran electron beam metrology apparatus.

Although the present disclosure has been explained in relation to itspreferred embodiments, it is to be understood that other modificationsand variation can be made without departing the spirit and scope of thepresent disclosure as hereafter claimed.

What is claimed is:
 1. An object table for holding an object,comprising: an electrostatic clamp configured to clamp the object on theobject table; an unloading unit configured to unload the object from theobject table; and a neutralizer configured to at least partiallyneutralize a charge of the electrostatic clamp; wherein the neutralizeris configured to at least partially neutralize the charge while theunloading unit is unloading the object
 2. The object table according toclaim 1, wherein the charge is a residual charge and/or a surfacecharge.
 3. The object table according to claim 1, wherein theneutralizer is configured to at least partially neutralize the charge byproviding a discharge voltage to the electrostatic clamp.
 4. The objecttable according to claim 3, wherein the object table is configured toapply a charge voltage to the electrostatic lamp to clamp the object,wherein the discharge voltage has a polarity opposite to the chargevoltage.
 5. The object table according to claim 3, comprising: a sensorfor obtaining information on the charge.
 6. The object table accordingto claim 5, wherein the sensor is configured to obtain information on aforce between the object and the electrostatic clamp.
 7. The objecttable according to claim 6, wherein the sensor is configured to monitora lift force applied by the unloading unit on the object to lift theobject from the object table during unloading.
 8. The object tableaccording to claim 7, wherein the sensor comprises a force sensor formeasuring the lift force.
 9. The object table according to claim 8,wherein the unloading unit comprises at least one lift pin configured topush the object away from the object table during unloading, wherein thelift pin comprises the force sensor.
 10. The object table according toclaim 5, wherein the sensor is configured to provide a measurementassociated with the object in a first state and the object in a secondstate, wherein, in the first state, the object is on the object table,and wherein, in the second state, the object is away from the objecttable.
 11. The object table according to claim 10, wherein the sensorcomprises at least one of a gap sensor, a height sensor, and adeformation sensor.
 12. The object table according to claim 10, wherein,in use, when the object moves from the first state to the second state,the measurement is representative of a movement between the object fromthe object table.
 13. The object table according to claim 5, wherein theneutralizer is configured to determine the discharge voltage based oninformation obtained by the sensor.
 14. The object table according toclaim 5, wherein, during unloading, the neutralizer is configured toreceive an updated information signal representative of an updated forceor an updated charge, wherein the neutralizer is configured to adjustthe discharge voltage based on the updated information signal.
 15. Theobject table according to claim 3, wherein the neutralizer is configuredto determine the discharge voltage based on at least one of measurementinformation, estimated information and internal signal.
 16. The objecttable according to claim 1, wherein the unloading comprises lifting theobject from the object table.
 17. An apparatus comprising the objecttable of claim 1, wherein the apparatus is one of a particle beamapparatus, an electron beam apparatus, a scanning electron microscope,an electron beam direct writer, an electron beam projection lithographyapparatus, an electron beam inspection apparatus, an electron beamdefect verification apparatus, an electron beam metrology apparatus, alithographic apparatus, a metrology apparatus, and an apparatuscomprising a vacuum chamber.
 18. The object table according to claim 1,comprising: a sensor for obtaining information on the charge.
 19. Theobject table according to claim 6, wherein, during unloading, theneutralizer is configured to receive an updated information signalrepresentative of an updated force or an updated charge, wherein theneutralizer is configured to adjust the discharge voltage based on theupdated information signal.